A few things.
A 5 gallon tank isnt big enough for neon tetras. 10 gallons would be bare minimum, and you would need 20 gallons to give the fish a good living environment.
While bettas and neon tetras are often considered a good match, this assumes a couple of things. Firstly, that there is enough space for both fish to co exist and have room to keep out of each others way. So we are back to 5g not being big enough. Secondly, that you introduce the betta into a tank of tetras not the other way around. The betta will have decided that the tank is his, and will be less likely to accept tank mates.
Adding 5 new fish to a tank that only has 1 fish in it is a big increase in bioload in 1 go. The microbes that cycle the tank and consume the ammonia grow to support the number of fish in the tank, so 1 betta. You add more fish and increase the bioload/ ammonia going in the water and those microbes need time to grow and catch up. Adding too many fish in 1 go is too big an increase in 1 go. Your system cant support it, ammonia rises, fish get sick. Increasing the bioload a lot, it takes a long time for the cycle to catch up and water quality can get very bad. Increasing the bioload a little takes less time for your cycle to catch up and the water quality doesnt get so bad.
Or you bought an already unhealthy fish, which is common. Dont expect every fish to be healthy and survive a move from the fish store to your tank. Quarantining fish is a good idea before they go in a main display tank so you arent introducing unhealthy fish.
So. We have 3 ways that the fish might have died.
- Betta killed it. Very possible.
- Poor water quality. This probably didnt kill the fish in such a short period of time, but is possible it will do so given another week or so.
- Already sick. Again very possible.
Do you know your water parameters? Preferably taken before a water change?
But, absolutely those fish cant be kept together in a 5g tank. Return the neons to the store if you can. If you cant return them you either need to get them a bigger tank or accept they will probably die in the 5g, or at best have a very poor quality of life. If you want to stick with the 5g tank then just keep the betta on its own. A snail for company should be ok. What kind of snails did you get? If you want to keep the betta with other fish then you need a bigger tank. Set up the bigger tank, cycle it, get the other fish in there making sure they are fish compatible with bettas, dont overstock the tank. Then add the betta last. Even then the betta might not tolerate other fish, so keep the 5g on hand to return the betta to should it not go well.
Who told you bettas and neon tetras in a 5g tank would be fine?
