Vet Q & AWhy is my fish tank getting rust colored algae after adding sand?
Sand substrate is always a problem as it promotes decay and anaerobic bacteria. If you need sand in a tank for a fish species (none of the ones you mentioned above actually need sand) you should not use it in the whole aquarium , but only in a small spot to accomodate the fish.
Rust-colored spots on the glass and gravel are due to diatom algae. They likely were brought in with the silicates in the sand.Diatom algae need the silicates to make a shell they carry around with them. Remove the silicates, and the algae should die. Removing silicates can be done with a silicate and phosphate filter, or, in your case, I would remove the sand completely and refill your aquarium with washed gravel. Algae also need lots of nutrients, so if you overfeed your fish, algae will increase. Feed only what your fish eat in one minute, no more.
Updated on September 24th, 2025
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