Kuhli loach problems, help!
I've had 3 kuhli loaches living happily in my 55gal for the past 5-6 months. In the past few days they seem to be spasming, and unable to swim properly. They lie together at the bottom of the tank contorting their poor little bodies around on the substrate, sometimes on their backs, sometimes on their sides. This is not behavior I have seen from them before, and I'm sure something's wrong with them. They're camped out underneath some plants so it's hard for me to get a good look at them, but their gills don't seem inflamed or red, neither do they seem to be gasping.
This tank is established, but over the past 6 months I've gradually transitioned it from a low tech non-planted to a high tech planted tank. I started CO2 injection more than a month or so ago. I started with dry ferts and an EI dosing schedule about 10 days ago - just before this started to happen with the loaches. My plants are looking better than ever, but apparently at the expense of my fish! I can't figure out what exactly is wrong though, my parameters are:
Ammonia, nitrite = 0
nitrate = 40ppm
ph = 7
gh, kh = 13, 7
phosphate = 0.5ppm
The first symptom exhibited by one of the loaches was that it was hanging out by the surface one morning - I guessed that the issue was low O2 and have since added an airstone at night, but the situation hasn't got better. The loaches are no longer at the surface, but they're still suffering.
Any advice most gratefully received.