Mollies and platties

Question
QUESTION: My husband and I just began in the world of fish. We started with 4 platties, 2 golden and 2 high fin, along with 2 emerald catfish.  That was in our 10 gallon tank, my husband decided that we need to upgrade our tank to a 29 gallon (God Bless PetCo and $1 a gallon sale.)  So we moved our fish we had over, plus on new baby fish (we didn't know they breed so quickly) and an addition 4 in the baby net.  After about 2 week we added 3 Serpae tetra.  After another week we added 3 gold dust mollies. Later that week I found a catfish dead with some white moldy stuff on his head. So we did a deep clean of everything checked water levels.The Alkalinity a little, we added more PHup. On a side note we struggle keeping that at the right levels. We also just introduced two emerald catfish.  We have several live plants. And we are using lifegaurd to help with what ever happened to our catfish.  My main problem is as we sit and watching them there is one platty (high fin) being chased all around by two gold dust mollies they always seem to nip at the underside of the fish...in the genitalia area.  Thanks for you help Im kind of worried about how much he is getting picked on

ANSWER: Jessica, I just want to verify what exactly you are asking so I can give you a proper response.

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QUESTION: Sorry i guess what i am trying to ask is why the mollies just keep attacking one of the platties, and how can i stop them?

ANSWER: My guess is that they are either the opposite sex and the molly is attempting to mate or they are the same sex and are being aggressive de to the other tank mates being opposite sex. Mollies can be aggressive sometimes to any fish. The catfish sounds as if it had a fungus of some sort according to what you described. IF the behavior continues than the only logical option would be to remove it from the tank so they are separated. Good luck. If you have any other questions feel free to ask.

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QUESTION: On another note all our water levels look good except our alkalinity.  I regularly have to add PH+ to the water but it will only help for a few days.  I thought it was fungus too (on the catfish) so we are using Lifegaurd to help clean it and did a deep clean.  Thanks for your help/

Answer
No problem, I'm always happy to help. I will suggest you do research on ph levels especially for the type of fish you have and the harm of ph levels not being persistant. It is very unhealthy and bad for ph levels to fluctuate. Most fish can adapt very easily to a ph level that is supposedly outside of its natural habitat. It is better for it to be a constant level whether or not it is outside its natural habitat rather than it snapping back to whatever it is before you add ph up or down. Just read uop on it. Good luck.