DO NOT, what ever you do, buy into the adivce being given to shoot for a perfect pH. A stable pH is much better on your fish than a perfect pH.
7.8 is NOT too high of a pH for most freshwater species. If you can get a 7.8pH or lower from your tap water, then I wouldn't worry at all with it unless I was keeping Discus or unless I was breeding.
Too many people try too many DANGEROUS things in order to obtain a perfect pH for their fish. The rapid and large swings in pH are much more dangerous in the short and long term than simply maintaing a stable pH.
The biggest problem relating to pH is when you bring home a fish from the LFS that has been raised and eventually sold from a tank with a pH that is WAY off from your tanks pH.
I would highly doubt that pH is a factor in ANY of your current problems.
If your tank has been fishy cycling for only a month, I would NOT suggest medicating. If the fish that you say has popeye has been in the tank since the cyling process began, it is highly suspect that the popeye is a result of the fishy cycling process.
I would suggest continuing on as though your tank is STILL cycling and continue with water changes and frequent, daily or every other day, water testing. It is quite likely that the fish you currently house will never recover from the effects of the cycling and medicating a still cycling tank could throw your cycling back to day one.
If you are paranoid about your tank somehow becoming contaminated with fungus or parasites, I would suggest starting over from square one and doing a fishless cycle.
Before I went and assumed the blame of the popeye or your other general fish health problems on Petco or the LFS, I could highly suggest considering the possibility that YOUR fishy cycling has exposed these fish to health problems that are just NOW starting to surface.
Hold off on the medication. Do your water changes daily and keep testing your water.
While the possibility does exist that your cycling process was NOT the culprit, the fact that you have sick fish coupled with the fact that you are only one month into a fishy cycle does bring into question past or present water conditions as being the main culprit.