Hi everyone. New on here and fairly new to owning fish, however my sister manages a pet shop and has plenty of fish and reptiles herself, and I've been assured I'm doing things right. I have followed all the instructions of the pet shop people along with guidance from online (mainly this forum). I have a 22litre tank with gravel and real plants in, and after 5 days of letting the water cycle, I added 4 Platy fish to it. One's a Red Wag, I have no idea about the other three. They all seemed pretty happy, swimming about with lots of energy, no marks on them at all and eating fine. After about 4 days they seemed less enthused about the food going into the tank, not swimming up to get it. When I came home from school five hours later the food was in a clump on the gravel so I removed it straight away with the net. My Red Wag is now hiding behind the filter almost constantly, and every so often has quick spurts of energy, darts up and down vertically quickly, then retreats. I have no idea what I've done differently, so I thought I'd leave it and see if things changed. After some research I thought she might be pregnant? Her stomach seems larger however I can't get near enough to tell if there's a gravid spot. Then, randomly, one of the other ones has died over night It seemed really sudden as it appeared healthy. I'm going to take a water sample to the shop to get it tested tomorrow (it's asap) and whilst I'm there, probably buy a birthing tank to be on the safe side? Any other thoughts as to why it might have passed away or what could be wrong with my Red Wag? I'm aware that they're tropical fish, but I'm guessing the shop adapted them. I'm thinking maybe the dead one simply didn't adjust as well? Thank you in advance.