If ammonia isn't spiked that there's a good chance it's just hiding out behind your rock work were it found a comfy spot. GBTA glow nicely under blue light. Go to Walmart or home depot and find a blue led
flashlight if you don't already have one. At night make sure you turn off all lights including moon lights. Use the blue flashlight to find him. Even if he only has a tip of a tentacle showing, you'll be able to spot it a lot easier than your daylight lighting. Can't miss something glowing bright green in your rocks. Good luck and hope this helps. It's worked for me several times recovering frags and 2 rainbow bta.