Timesdragonfly

Hello, We lost one of our Emrald green catfish last night, he got really dark in color, almost black and then he died. Now, the other three are getting dark in color as well and seem less active. Does anyone know what this may be? How do we treat it? Please help!

We haven't tested the water in about a week, i've had the test kit at work w/ my new betta tank. I'll get water test results posted tomorrow. I can tell you that we've been doing once a week 20 percent water changes, and the readings last week b/4 the water change was Nitrates 15, nitrites 0, ammonia 0. So all was good there.

Thank you for any advice!

~J~