HELP MY STINGRAY!!

[hereticlosmorte]

whoa, only 150g for that stocking? combined with the water changes, that sounds like your water would be no where near perfect values.
sounds like everything is crammed tight in there.
what type of rays are these, as teacup is not a type. motoros, retics, scobinas?

the ray could simply be getting bullied by the other fish and is stressing. add in less than optimal water conditions and it will go from bad to worse rapidly.

 

[Just Toby]

With that stocking I expect the ph is beginning to drop (this value cannot be zero) and I also expect the nitrate is high, as per my original reply.

I expect that they have reached a size and stocking density where the parameters are beginning to shift a lot and the peer group pressure is causing other issues.