Hoplos Dying

I acquired 6 hoplos 10 days ago. Within 3 days 4 had some sort of tail rot and died. The fifth one lasted a couple more days and then passed. The last one is currently on his way out. These catfish are supposed to be bulletproof and I am at a loss as to what happened. My parameters are normal, no ammonia, nitrite or nitrates. pH is around 7 and temp is 78. I have some plants without any CO2 injection with a black sand bottom. These were the only bottom feeders in the tank besides an old bristlenose pleco. All the other midwater fish are fine and haven't shown any signs of fin rot. Pictures of current hoplo:



Picture of tank:


I have an eheim 2217 and a fluval 406 for filtration. Anybody have any ideas?

 

Zero nitrates?

 

You would be amazed what a pothos plant will do when you let its roots in a tank.

 

I heard they were great for nitrates but didnt know they were that great lol

 

Last one has died. It only takes 2-3 days from start of disease to death. Nobody has any insight into what happened?