4 sudden deaths in 2 days, what am i doing wrong?

Question
QUESTION: Hi Rachel, my name is monika i have a question for you, but I'll tell you the
details first.i started my tank last saturday. i have a 20 gallon tank and 19- well 15 fish now.i check the
ammonia and the pH regularly but before i started doing tests, one of my
gouramis and one of my red velvet swordtails died. we knew there was something
wrong with the swordtail but we weren't sure what, he wouldn't go near the rest
of the fish and would just stay in a corner. The gourami was acting very scared,
when i would feed the through the feeding ring, he would rush to the other side
of the tank. So we waited to se what would happen tomorrow.but before the next
day, i noticed my black and white molly was swimming oddly, when i first put herin the tank she was starting to get harassed. i looked on the Internet the said
it was an alpha battle to see who was the better female. The next day when i
woke up, i saw poor molly stuck to the filter, motionless. Then i left to go
shopping, i saw one of my very very hyper zebra danios had unfortunately
deceased. Afterwards we cleaned the tank and all fish were normal. But i was
wondering if i had changed the water earlier, if i could've saved my fish. well,
all fish are good now, but please answer why 4 of my most beautiful fish had
died. bye.
p.s- is an air pump nesscesary?


ANSWER: Let me address your more simple question first, the air pump. Well, I imagine you mean an air stone providing air bubbles from an air pump, right? Assuming this, no you do not NEED an air pump but it gives the tank alittle more oxygen and also gives decoration. If you had goldfish, I would definately say YES use an air pump because goldfish require more oxygen than regular tropical fish. However, keep in mind if you have more fish than you should, which 19 for a 20 gallon is overstocked, you should use an air pump to make sure there is oxygen. I assume you have a filter? If you have a managable amount of fish in the tank a filter by itself would be just fine for tropical fish. Ok, now for the dead fish. Well, since your tank is new, perhaps the fish were simply too stressed by the relocation to your tank. Or it could be they had a disease from the store and just were overtaken by it due to stress, or perhaps one of the other fish is a bully and killed them either through stress or actually picking on them and causing injury. There are  so many ways a new fish tank can have dead fish it just all depends on what happened how it happened, where you fish came from and so on. So many things can go wrong in the new tank. You should not have more than 10 fish in your tank, depending on size. I know it is tempting to get every single fish you like but it makes for a dangerous situation and in the end leaves disapointment. Let me know if you have more questions and I will do my best to help you.

---------- FOLLOW-UP ----------

QUESTION: thanks so much for getting back to me so quickly. i do have another question though, i bought my first fish at pj's pet store in pickering and then others in petsmart near homedepot. Is there a particular reason why most of my fish that were from petsmart died, the velvet swordtail was the only one from pj's pet store that died. The other three, (the gourami, the zebra danio and the back and white molly) that died were from petsmartt, is it just possible that i bought sick fish, or does one store treat their fish better? odd question, i know. Well ever since the deaths, i have been so paranoid with the fish, always thinking when i check, there'll be a dead one again. well thanks for your earlier answer!
goodbye

Answer
Not an odd question at all. I do NOT ever recommend getting fish from a chain store. Either the people caring for them have no idea how to properly do it or the fish themselves are not from strong lines nor properly quarantined. Indeed, the possible problem could have started from the chain store, in fact that is highly likely. Please, dont get anymore fish from the chain store. They are great places to get supplies but NEVER fish. I get alot of questions about buying from chain stores and always say the same, stay away from thier fish! Hopefully, the local private store is a good one and has a good variety. If not, look around perhaps there is one not far away. It would be better to travel alittle than get a bad fish. I hope I have helped. Let me know if you need something more.