Bristlenose keep dying need help

[samrex]

Hi guys

I have a 56 Litre tank 

I have lost a bunch of bristle nose and other fish as I had some issues when using continuum fraction, for some reason it was causing my PH to drop to 6 or lower a day or 2 after water changes, but have now switched to prime and my ph is stable.

Tank is  now 4 months old with a PH of around 7.5  and KH is 3, Ammonia, Nitrates and Nitrites are all zero

Substrate is small black gravel and i have some plants and small amount of driftwood.  4 tetras and a pair of bettas and some cherry shrimp in the tank all alive and healthy. They get fed once a day and eat all that is fed within 2 minutes except the Aqua One Vege Wafer Food i put for the bn before bed that I remove the next day if any is left

My problem is my bristle nose keep dying. I put another 4 albinos bn in around 5 days ago 5-6cm in size and 3 have died.

I have noticed the belly internals looking through the skin look black after they are dead and There is a small amount of brown algae in the tank but other than that I have no idea on whats causing them to die off.

Any advice is most appreciated as i mainly set this tank up for keeping some BN

Thanks in advance