Fish dying after water change

Question
I am having trouble with my 55 gallon tank. It has the filter that hangs on the back of the tank. On March 8 I brought home 7 fish from petco. Placed them in the tank after setting the pH to 7.5 like I was instructed by the guy at petco. I got a rainbow shark, 2 mollies, a catfish, and 3 glo fish. They all did great other than my mollies not eating for a couple days, so I went to the local pet store and the lady told me I needed to add a little aquarium salt for the mollies and stress coat. So I bought some and added the stress coat and salt. Within 30 minutes after adding them my mollies started shaking from side to side. I was worried that the stuff I had added was causing more harm than good so I did a 30 percent water change and they were better within the hour. Since then all my fish were doing great and the mollies began eating. Six days ago I add a male betta and a pleco. Everyone still was doing great, other than the rainbow shark picking on the betta. Then three days ago the water was still cloudy so I did a 50 percent water change, which now I know was too much, I had 2 glo fish that started swimming upside down and died during the water change and now I have a sick molly that just sits on bottom of the tank and every once in a while will swim around for a couple minutes but then returns to the bottom of the tank. There was a little difference in temp but it stayed in the green area on the thermometer. Today I went to the pet store and got a ammonia test and tested the tank and it tested a 4.0. Which my understanding is pretty high but it also says on the instructions that new aquariums can spike to 4.0. The pH of the tank is currently 7.6. I have a gravel vacuum but the lady at the pet store told me not to use it because I might get ride of the good bacteria and she also sold me stress enzyme the day before that I added to the tank to add good bacteria. I don't know what to do about my molly. I dont like seeing her suffer at the bottom of the tank and not getting better. The lady at local pet store thinks I should just leave the tank alone but that was also before I knew the ammonia is 4.0. Is something I can do to help her without hurting the other fish. The stress enzyme says use it on the 1st, 7th and 14th day. Should I add more? Use the ammonia reducer I bought? Gravel vacuum the tank? I just feel that my fish were perfectly fine until I did the second water change. Thank you so much for your time.

Answer
Hi Rebecca,
I'm sorry to hear about your fish issue. :(

I think the simple answer to your problem is your aquarium simply has not had enough time to cycle and ammonia will naturally start to spike until the beneficial bacteria can establish well-enough to handle the ammonia to keep the fish safe.

During that time, fish loss rates are very high. But you can do something to prevent so much---or prevent it all together.

Water changes are the best thing you can do for your fish now. 4.0 is deadly high for an ammonia reading and I would start doing daily 50% water changes. Water changes will not hurt the fish provided you always make sure the temp is equal to or just a bit warmer than your aquarium's and always use water conditioner. I would only lightly gravel vacuum but the main thing is to get the polluted water out and dilute it as much as possible. It helps to use a water conditioner like Amquel+ or Prime because they will help neutralize some of the ammonia in the water and are great to use along with your water change.

Try to get your ammonia down as much as possible with big water changes and try to monitor it by testing everyday and taking note of how your fish are behaving. Also, it would help to know your 'nitrite' reading as well as 'nitrate' reading because then you can determine just how far along your aquarium is in its cycling.

The stress enzyme is OK to use. I wouldn't mess with the pH at all however, always beware the petstore advice. Its best to leave your pH where it is because fish naturally adapt well to varying levels of pH and trying to constantly adjust it can stress them tremendously. With the ammonia reducer, is it 'Ammo-lock?' that's a good one. Whichever it is I would go ahead and use it but don't use it as a substitute for water changes.

I know the ammonia must be getting to your fish and weakening them bad. I hope all goes well and let me know if you need anymore help!

Karen~