Slime problem

I'm new to this subreddit and new to aquarium setups, so excuse my ignorance.

I'm having a recurring slime issue. You can see it here

My setup looks like this.

40 Gallon Freshwater tank 50 Gallon Aquaclear filter on the left 10 Gallon Carbon filter on the right

• 1 Apple snail
• 1 Red Crayfish
• 2 Algae eaters
• 1 Angel Fish
• 1 Huge catfish Just like this one
• 3 really small minnows
• 2 Big Gold Fish (They were suppose to be feeder fish for the crayfish, but the crayfish is too nice, so they ended up with the herd)

I wanted to go with a sand base, so I used pool filter sand and washed it pretty thoroughly. New water and the tank looked great for a week then started to get cloudy. This film started building up inside the tank which causes the filters to slow down and eventually if left will clog.

I've been forced to do a 40% water change once a week and a 60% water change every couple of weeks. Fully clean all the filters and the tank will go back to looking good for a week and this cycle will repeat.

I have some ceramic ornaments and 1 large wood log at the bottom, purchased from a pet store.

I'm not quite sure what's going on.

Any help would be appreciated.