Problem In Mature Filterless Tank

[Mikaila31]

So the problem is for the last 1-2 months there has been an unmanageable amount of mulm building up in my 15 gallon filterless tank. This tank isn't normally crystal clear, but its pretty close to it. I can siphon a bunch of it up and do a 50% water change and have it already building up 24 hours later. Looking back I've lost a number of fish to some slow chronic disease probably parasitic or bacterial. Basically they up and die without any outward signs or symptoms. Water quality has been fine the whole time. My cherry shrimp and bamboo shrimp are all healthy and happy. The fish issue may or may not have anything to do with the mulm I'm experiencing. My plants are growing pretty normally for the most part.

This tank was first setup about a year ago. It has a layered substrate with regular topsoil covered with sand. Three months ago it was moved. I drained it most the way, left plants and hardscape intact and moved it about 20 miles away. It was fine for about the first month. Had about 24 small fish in it. Currently theres less then half of that.

Before it was moved

a week or two after it was moved-kinda over grown

These are from yesterday, 24 hours after a waterchange and heavy pruning. The build up will get worse as the week goes by. It seems kinda 'sticky' and can build up on leaves in a moderate current.

So any ideas or suggestions on what has happened here? I can't figure out the source of it. Also how to fix it short of getting a filter lol...