Three fish in one month is a little too fast as far as stocking your tank. You have not given your bacteria time to increase and handle the bioload. This is probably resulting in a nitrate buildup as your dentrification is behind the bioload. Diatoms, algae blooms, and red cyanobacteria thrive on the nitrates.
It is better to add fish very slowly to allow your tank to properly mature. While diatoms, algae blooms, and cyano are a regular part of the tank maturation process overstocking will only exacerbate the situation.
Go slow and let it grow.
HTH
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Currently tankless.
20 Gallon mini reef with mated pair of Maroon Clowns given to Rougiem!
80 gallon reef given to Rougiem/Wooster HS.