What is wrong with my fish?

I have a 3 month old Molly that hasn't been acting well for the past week. She lives in a 5 gallon tank with 3 other baby Mollies - salinity 1.010, pH 8.2, ammonia 0, nitrites 0, nitrates 5, temp was at 72 (I read fish are more disease resistant at lower temps). She swims and wobbles at the surface of the water and is unable to swim down, listless, and has difficulty eating/loss of appetite. Research indicated possible swim bladder disease (bacterial infection). I've used Erythromycin to successfully treat gram-negative and gram-positive infections in the past. I read that it would treat swim bladder. I've been treating with Erythromycin for the past 5 days (requires 10 days). The wobbling stopped, but still not much improvement. I found out that raising the temp to 82-84 helps control disease. I've been steadily raising the temp over the past 3 days and it's now at 80. She possibly seems a little better. I have already lost 3 other fish to similar symptoms resembling whirling disease.