8g Biocube nitrates up

[Proteus]

I would think that new fish and over feeding are the issue
For the cheato from your friend I would use a hob style filter with a reading lamp over it. Just replace the filter media with cheato
There are skimmers or dyi that could suit your needs but are unnessecsary in your case.
Spot feed your fish so you don't have organics going to waste in your tank and keep up with water changes

You could also look into dosing a carbon source but it would have to be very percise with a small water volume