Fish keep dying...

I've been a fish owner for 3 years now. I have 2 malawi tanks and this one.
- Had 13 fish in 55gl - varoius haps and peacocks, all male, roughly 1.5" each
- NH3/4 - 0
- NO2 - 0
- NO3 - 10ppm
- PH - 8.2
- KH - 9 degrees
- Temp - 79.5
- 10% water change weekly
- Using Hikari Ultimate water conditioner
- Fishless cycled - added fish 24 march.
- Water parameters have been steady for a month since I've got the fish.
- Feeding once a day varying from Frozen life line Omega 3-20, Hikiari Cichlid bio gold plus and Cichlid gold sinking formula.
- Ehiem classic 2217 filter - haven't changed any filter's since starting tank. was going to next weekend, that would be 5 weeks, I didn't want to disturb the delicate bio balance to help establish a good tank.

I've lost 5 fish since Thursday night, all peacocks, now soon to be six, only this one's a hap. Not sure what's going on. I've tried to examine the fish that died...the only factor that looked the same was a semi bloted belly. I watched behaviors of all the fish in the tank, the ones that look like they are gonna die usually sit on the bottom, look like they are having trouble breathing and have a slightly swollen belly. I cut back on food as soon as I saw the first fish die, I only feed them once every other day now. Plus I've been looking for signs on parasites to see if I should add clout...nothing has lead me to that thought process yet.
My other fish seem to be 100% normal, Eating and swimming fine.

any thoughts??