Why does the fish, that swim happily at the aquarium shop, without obvious reason die immediately or a few days after they reach our house or die one by one in a few weeks until only one fish is left? I have spent significant amount of money for this. I do not want anybody else to repeat the same mistakes.

My aquarium air valve used for dripping

Acclimation (IMPORTANT)
When buying fish, get the water from that fish tank itself. Do NOT take the water given by the shopkeeper. This is to avoid shock that is due to the water difference. It is impossible that shopkeepers do not know this. So I guess that they want the fish sold to die faster so that they will have more business. The shop that I have seen doing this is no longer in business now.

When reaching home, this simple procedure MUST be followed to accommodate the fish to the water of the tank. If it is not done, the fish may die after one week. This is called drip acclimation. Pour water slowly from the plastic bag together with the fish into a pail (about the same volume with the plastic bag). Use aquarium air tube to siphon tank water into the pail. Adjust air valve (buy it if you do not have it yet) at the end of the tube so that water drips instead of flowing. Put a net over the pail to avoid the fish from jumping out. Let the water drip until the pail is full. Finally catch the fish with a fish net and release it into the tank. Pouring the fish into the tank together with the water is not recommended because the water from the fish shop may contain some germs. If the fish are gasping at the water's surface (sign of lacking oxygen) in the dripping process, stop the procedure and go straight to the final step.

Water change (IMPORTANT)
The change should be less water more frequency especially for less hardy fish. Changing big quantity of water at one time may cause the fish to get physical shock. The target is to let the water to achieve biologically and chemically steady state.

My female platy in dropsy

Daily partial water change is essential for fish survival even though the tank is crystal clear. The fish at the fish shop looks vibrant due the frequent water change (always replenishing water into the tank after sales of fish). If the water change is not enough, the fish will have dropsy. Fish dropsy is a disease that makes scales stick out; it cannot be treated but it can be prevented. My female red platy fish got dropsy and died. For example 20 liter (5.3 US gallon) needs about 1 liter of water change daily. Wash the filter element of external filter by immersing the filter element in the water taken out of the tank. Beneficial bacteria on the filter element is preserved if the filter element is washed with water from the tank itself. Filter element if not washed may become a pollution factory. After washing, the brownish / greenish wash water is then used to water potted plant if any. I add two squirts of dechlorinator to one scoop of tap water (approx 2 liters). It is better to pour new water into it so that the fish will have less shock. Do not just immerse the scoop in the fish tank because the sudden big addition of new water may cause physical shock to the fish. I did that once and the next morning one Rosy Barb and four Gold Tiger Barbs died.

Keep one bucket of water and let it age for one day (24 hours). Then on the next day do water change (also wash filter element if you want) and refill the bucket of water. Then continue on to do the same. Dechlorinator is not needed in this case.

If you are busy then you should change a quarter of the tank weekly, provided the tank has plants. Weekly water change is the minimum. If you cannot even do the weekly water change then you should not keep an aquarium. If you do not change water at least weekly, then the chemical difference between new water and old water will be high during the next change and causes stress in fish when the new water is poured in.

Major water change usually is unnecessary if water is partially changed daily and filter element is washed periodically. I do major water change when the substrate needs cleaning, some suds forms on water surface or a fish has died. For major water change, do NOT change all of the water and clean everything. The most is to take out the water until the level that is just enough for the fish to breath without evacuating the fish. Right before changing water I stir the substrate (of course the part that does not have plant). If the tank does not have algae eater, rub off the algae (if any) on the tank walls. Thus some alga cells and dirt can be also removed during the water change. Stirring the substrate is to aerate substrate and to clear away accumulated harmful matters. After major water change, I start doing daily water change one day after the next day.

When the water level decreases due to evaporation, do not just top up because pollutants are still in the tank. Some water change is still needed.

With the same frequency of water change, bigger tank has higher survival rate than smaller tank. Thus, try get the biggest possible tank by considering the budget and the space you have.

Nitrogen cycle (IMPORTANT)
For new tank add just a few hardy fish (I recommend Black-skirt Tetra) first to produce some ammonia to let the nitrogen cycle takes place. Ammonia buildup multiplies Nitrosomonas bacteria that convert ammonia to nitrite and they will colonize the tank if there is constant supply of ammonia. After Nitrosomonas fully colonizes in the tank, Nitrospira that breaks down nitrite to nitrate will multiply. They will also colonize the tank if Nitrosomonas are not killed by mishandling. At least one month is needed to achieve full cycle (ammonia to nitrate). Only then fish that are less hardy can be introduced. During bio-cycling the tank, small and frequent water change is as necessary as usual time. The water may become cloudily white or green during the period. Just let it be. When the water becomes clear, the nitrogen cycle is complete.

If many fish are added into a clean tank, the ammonia spike (relative to its previous clean non-ammonia state) will kill the pioneers. If the ammonia is very severe, red streaks and spots on the fins and tail, and even black marks on the tips of the fins and tail which are ammonia burns, can be seen. Some of the fish in the tank may die immediately, but even after the problem is taken care of, damage to internal organs may kill some fish weeks later. For this one, prevention is better than cure. Again:
1) Do not house too many fish.
2) Do not feed too much.
3) Change water frequently.
4) Provide more oxygen (by more air exchange) so that beneficial bacteria can do the job.

Feeding
New tank owners tend to overfeed the fish. When we go on vacation, our helpful neighbor may overfeed the fish. Four-year-old kids always overfeed the fish. If two persons feed the fish, the chances of overfeeding double. Long feces is a sign of overfeeding. How overfeeding will kill the fish:
1) Rapid health decrease. Fish dropsy (a disease) may happen. It is unnatural that food is always available. Occasional hunger can promote health.
2) More ammonia in water due to more excretion.
3) Unconsumed food pollutes water.

Feed only one time a day, preferably in the morning when the fish in natural habitat starts to look for food. This is another reason that the fish at the shop looks healthy. The shop cannot afford the cost of fish food to feed twice a day. Drop in fish food until the fish do not eagerly chase for the food. After doing this a few days, you will know to estimate the amount to drop in. Later in the day if you feel like seeing them eating, you can drop in just a very little food.

While avoiding overfeeding, be cautious at the same time of underfeeding. Watch out the food pellet size based on the fish size. I like to have tiny pellets (less than 1 mm diameter) because they are easier for bigger fish (3 inch) to digest and easier for smaller fish (1 inch) to swallow. Fish that is neither surface feeder nor bottom feeder (e.g. black-skirt tetra) must not be fed with big pallet because the fish either cannot eat it or tries to swallow it (and has problem later). If the filter disturbs the feeding (like sucking some food into filter), switch the filter off temporarily.

If nobody helps feed the fish during vacation, put in a holiday food block. It is even better if an automatic food dispenser is used.

If the fish look sick and you are sure that underfeeding is not the reason, then you should even feed the fish only once every two days. Starvation heals, even for humans, dogs and cats.

The food pellet should not contain artificial colorant and other chemicals (such as synthetic compound that is said to enhance the color of fish).

To further enhance fish health, provide some unprocessed food (frozen bloodworm, live brine shrimp etc) occasionally. If your area has mosquitoes, put a small cup of water outdoor. When mosquito larvae are visible, pour the water into the tank. The fish will be happy to have this treat. Wash the cup and repeat the same thing.

Water to put into tank
Let city / pipe water sit out for three days or use dechlorinator. If pipe water contains chloramine (ask the the local water company), then sitting out is not enough but dechlorinator that can neutralize chloramine should be used. Well water may contain harmful gases but there is no conditioner in fish shop for it so let it sit out for a week. Do not pour distilled water into the tank. The fish can be killed. Distilled water absorbs things (mainly carbon dioxide) from the air. Only outdoor soil can neutralize things in distilled water. Rainwater is distilled water too. Boiled water is no-no too.

Plants
Plants provide oxygen and reduce waste. Some fish (for example pleco) may be spooked to death when the light is switched in the dark if there is no plant for the fish to hide under.

Quarantine the plant taken from lake or river for a few days in mild saltwater before putting it in the aquarium as the it may have harmful bacteria or chemical. The plant may also have snail's eggs that may hatch and infest the tank. The plant bought from the shop should also be quarantined for one night to clear off excessive fertilizer before planting them in the tank.

Devil's Ivy (Epipremnum aureum) is a very hardy plant that can survive with just water. But do not put it into aquarium. It is poisonous and will slowly kill all the fish. I did that once and none of my guppies survived.

Companionship from the same species
Find info on the number your fish prefers to be. This minimizes mental stress of the fish. Some fish definitely do not prefer to be alone. However for some fish although there is companionship but the number is not enough, the fish will become aggressive towards its own kind. For example I once had two black female platy fish. One picked on another until it died. I think if I had three (one male two females, three females or three males), this would not happen. For the type of fish that I cannot recognize the gender, I will still keep at least three.

Trace elements (invertebrates only)
Copper kills invertebrates (shrimp, snail or clam). Do not put aquarium under an air-conditioner to avoid water dripping into the tank. Air-cond contains copper pipe. Also see whether the fish supplement / medicine contains copper.

Others
The bigger the tank is, the easier the survival is. I recommend the tank should be at least two feet long and one feet wide.

All fish like cave. Put in a short opaque channel or pipe that is just big enough for fish to swim through. Some fish will rest in there. If there is enough hiding place within the plants, then a cave may not be needed.

When buying fish, do not let the fish shopkeeper choose the fish. Avoid fish that has injury (torn fin, missing scale etc).

To add more than one type of fish, observe compatibility at the fish shop itself. When you see an interesting fish but you do not know yet the compatibility with your existing tank inhabitant, then do not buy it first. I did not know puffer fish would harm eat snail and shrimp when I bought it.

After stocking a tank, observe closely if there is any bite injury on any fish. If yes, isolate the fish with an aquarium divider to avoid casualty.

Do not overcrowd the tank. If the fish are gasping at the water's surface, it is a sign that there is not enough oxygen in water for the number of fish in the tank. My own reference for fish number estimation is the tank bottom available area. I ask myself this question: "Is there enough area for every fish to sleep?"

Add new fish only when there is no dying fish or other problem for a significant period of time (e.g. a fortnight). Add one type only at a time so that we can observe compatibility.

The fish needs at least a few hours of light a day to stay healthy.

Chloride in the salt neutralizes nitrite that appears especially in new tank. Salt also helps fish that have scales build their protective slime layer. But fish without scale or baby fish cannot tolerate salt. If water change is maintained, nitrite and disease will not be an issue so it is NOT necessary to add salt. Salt should be used only to treat ich. To treat ich, try not to use table salt because it may contain unnatural additives undesirable to fish. Buy salt from aquarium shop.

Do not spray insecticide at home.

Wash filter media immediately when there is dead fish.

For goldfish
I specially mention goldfish here because it is the most popular aquarium fish.

The goldfish may suffer SBD (swim bladder disorder) in future if it always swallows many pellets at a time. The pellets will also expand in the stomach. Overloaded stomach hurts the swim bladder. You can give one pellet for one goldfish at a time and you can do this more than one time in a day whenever you go see the goldfish. By doing this, the goldfish will also be trained to come to you.

Goldfish poops a lot so the gravel layer may trap a lot of poop that is harmful for the tank environment. Also the goldfish may be accidentally choked by the gravel. Thus it is better not to have gravel in goldfish tank.

Fish with sucker mouth (e.g. pleco) will suck on goldfish. It may harm goldfish physically. And the goldfish will be under stress. Stress kills. So do not keep them together.