How to keep corys?
Hi everyone! I have a question about keeping corys. My boyfriend and I have a 20g long tank with some platys and tetras in it -- and they're super hungry all the time. We've unsuccessfully tried to add some corys into the tank twice. We buy the healthiest looking out of the bunch (my boyfriend knows more about this kind of thing -- he's a budding aquarium enthusiast), quarantine them for a week -ish (not sure on how long he puts them by themselves since the tank is at his apartment), and then add them to the 20g because (a) corys are really cute; (b) we originally bought them to pick up the leftover food.
Except when we get the corys, they have all died :( He's made sure to feed them adequately and distract the other fish while feeding, but we've found all of the corys dead a few days later. I used to have a betta + 3 corys in a 10g, but I wanted the corys to have a bigger tank and more cory-friends, so I gave them to him. We added my 3 and another 2 into the 20g. One of mine died (they were all very healthy-looking; luckily I saved the other 2 before it was too late!) and so did the other 2 from the fish store. We are lost as to why since corys are pretty hardy fish. I know they like to be kept in groups of ~4-5+, so keeping 2 together is temporary. :)
Anyway, we're going to use an empty 29g to combine fish tanks in August when we move in with friends. I love my little corys and want them to have more cory-friends and a bigger tank, but I also don't want to send them to their deaths by adding them to the fish tank. So far, they're doing great with my betta fish and get along well. (But I think he'll be happier by himself :P) All the tanks the corys have been in have bubbles, a heater, and a filter. When all the corys were together with the platys and tetras, we made sure to give them both sinker food (fish pellets) and flakes.
Sorry this is long and if this is a stupid question. I just want to figure out what I should do before we move them. Thanks!