Unlikely to be Bristlenose. The tank I had set up for years (Mk1 fish house) was up to 13 loach, up to 12 glass catfish (Kryptopterus) and a Bristlenose.
That stayed like this for years, until I moved, I gave 12-13 loach to a friend during the downsize. Lid had feeding flap removed as tank was on top shelf of stand (pain in the old doo dah at feeding time) it had the lights in but was open full length along the front, with a pair of cut outs at the back.
Maybe it's the upper end of your No3. 30ppm was high (I thought)
It could be they don't like nitrate. (I realise plant needs are different)
Loaches are typically considered nitrate haters.
Co2 could be a cause, I've not used it but it can cause massive pH swings, even without co2 addition, the level of co2 in the water can shift pH by great amounts over the course of a day.
(Recent loss of corydoras for me, exactly ties back to this, adjusted photoperiod no more losses but they died in tank, 7.4-8.2 was the change, this is deadly)
Is it 7.2 stable? (Test in the MINUTES before lights on, AND in the MINUTES before lights off, that will give you the daily range for your tank.
(It may be 6.8-7.8 or something, depends on Kh, O2 (air stones) and other environmental factors, it could be much more?) but it's just a guess?
I had a well planted tank that had no co2 addition, but when the lights went off I turned an air stone on to keep the level from changing. (Drives off co2 now being produced)
So maybe, as the lights go off, added co2 (not used up) combines with that now being produced by the plants and fish, this will acidify your water, the speed of change will depend on the level of co2 and rate of gas exchange etc. (that's basic but close enough)
If the pH is moving too much, just counter time an air stone. This should help.
(Measure Kh, this will let you know roughly how much co2 you have, you may have a colour check or some co2 testy thing? If so this can be used to guesstimate Kh)
My substrate was a mix of regular grade gravel up to pebbles
(large, er beach grade! To fist size)
Yes, they do bury themselves! Up to the eyeballs! Silly fish.
It could be the food thing? I can confirm they (and most if not all fish) will learn where the food comes from, I feed the tank area though, so that could be it?
(General scattering of dry foods is more natural, that way they have to search, similar to baiting an area, got that from a fisherman, frozen stuff dropped in front of filter return line, makes it wriggle like its alive! And it scatters it around)
Whew, complex! Hopefully it gets sorted, I still remember seeing my first bichir crispy on the floor, that was years ago! (Maybe 9) but I still see it, and I still feel responsible.
Good luck! (Seriously, because I hate this type of thing, it is gutting)
