12 bls of rocks 10 gallon tank
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You need to get yourself some test kits.
Nitrate and Phosphate. Nitrate I would go with Salifert or Red Sea... Phosphate I would go with the Hanna Checker.
But Algae grows when there is an excess amount of nutrients (Nitrate and Phosphate).
You have a saltwater tank with a hang on filter, not the best setup long term to be honest.
Regular water changes will be your best friend for that tank.