Tank problem

Hey guys.

We have been having some problems with our axolotl tank since we moved recently.
It is a ~20 Gal tank, and the substrate is some very fine gravel, along with that it is decorated with rocks/driftwood to create some hiding places and a couple of plastic plants.
There is an internal filter running which has never bothered our Axo before in the tank.

The problem is that the tank gets extremely dirty and foul after only a couple of days, there is some brownish algae/bacteria covering everything in the tank, and on the water surface some white "slime" accumulates. Very frustrating and i dont know what is going wrong, its the exact same setup as we had before moving.

Our Axy has been fridged for the time being, while i sort out this problem with his tank.

I would appreciate any ideas from you guys, thank you!

What are the test results
Temperature (average)
Nitrate
Nitrite
Ammonia

How deep is your substrate?

How long are the lights on each day? Is the tank near a window?

What type of media do you use in your filter?

Hey

Well, the tank parameters were fine a couple of days ago, and the temperature ranges between 17-19 Degrees Celcius during a day.
The only thing i can think of is that it is for sure exposed to a LOT more sunlight in its new location than it was before, do you think that may be causing the trouble?

The substrate is about 2 CM deep, and the filter media is a regular sponge, driven by an airpump to increase aeration.