What you likely have happening here is acidification from the detritus that is definitely occupying the space beneath your
UGF. Decaying matter over time will acidify the tank and cause a drop in pH. This in and of itself is not necessarily bad, but trying to bump it up, only for it to drop, is stressful for the fish, and can cause death in some cases.
Just to be sure, what are the ammonia, nitrite and nitrate readings?
I'd remove the uplift tubes and get a flexible hose of some kind and start a syphon and try to suction as much gunk as you can from underneath the
UGF. They unfortunately do require maintenance. In the meantime the best way to keep your pH a bit higher is to do more water changes, like maybe twice a week, until you get a handle on what is going on.
pH is very dependent upon
KH, or alkalinity, so do you have a test for that? If so, I'm interested in what your
KH readings are. As long as you have at least 4 degrees or so then we can assume that removing accumulated debris and increasing water changes will solve your difficulty.
I'd not take any corrective measures until you post back with water parameters, though, as it might impact how you handle it.
Good luck!