Isn't the fx5 is rated for 700 g? I've had it running by itself for 2 years and never had a problem. Water params are always good. I change 50-70 gallons a week. Never a spike. I also have one power head
I beleive it's rated for more like 400 gal but that's the mfg rating. It depends what your livestock is. I'm not in this to come off as a know it all or pick a fight. I just personally don't feel it's adequate for a large Ray tank. If it works for you that's great.. I just know it wouldn't work for my big Rays. I hope the OP can figure out the problem. If it's not water problems then It leaves parasites as a very possible culprit.
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