4 times filter turn-over and good waterstats, so that shouldn't be the issue
Stocking is 78 inches of fish, and you have about 36 gals. Stocking is quite heavy, at bout 2 and a bit inches per gallon, but the filter should handel it. Do you have any other tanks to allow you to run an experiment?
If so, try moving out the plecos for a couple of weeks and see if that makes a difference. Most things in there are low waste producers, hence why I'm not quite saying strait away that it's stocking, but IME bristlenoses are very messy, and may well be pushing too much waste into the system for it to handel.
It is a long shot, but is a possibility... Not likely to be the caurse though
I don't suppose you have a phosphate test kit, or an LFS that will test phosphate content do you?
Also, are you using liquid drop tests or strips?
What's your water change regime like?
All the best
Rabbut