Why does brown algae keep returning to my fish tank decorations?

Thanks for reaching out about Corn and including photos. It appears as if this is a type of algae. Try washing in hot water. Overgrowth of algae can potentially cause some problems and foul water quality. Algal overgrowths often associated with too much sunlight. Good luck.

Check the water chemistry using a home test kit or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Perform partial water changes to stabilize chemistry and/or to improve water quality. Rinse the filter media in tank water, not tap water, and replace it. Use only conditioned water that is temperature matched to prevent stress or shock. To limit algae growth, maintain pristine water quality via filtration and regular water changes. If possible, turn up filter flow to keep water moving. If this stresses the fish though, do not do it. Do not overfeed and remove any uneaten food promptly. Limit lighting to 8 hours daily and closely monitor water temperature as well. All these can promote algae overgrowth.

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.

There are different ways to control algae accumulation inside the tank: Limit the nitrate accumulation in it (regular water changes is the most common method of removing nitrate), limit the lighting period in the aquarium (4 hours a day) avoid overfeeding. Try Algone, very effective against nitrate, it is not an algaecide but it can control indirectly algae. If all these methods are not effective, algaecide are then needed.

Algae is a constant in an aquarium. The smaller the tank the harder to keep it clean. Bleach will kill any fish you put in the tank after using it, and it will just come back anyway. Frequent partial water changes will help, but manually wiping the tank is the only way to keep it clean. An algae eater can help but usually not a good idea with a beta fish.

The green you see is algae. These like nutrition-rich water and lots of light. So you are probably feeding your fish too much and they are not eating all the food. Also there may be too much day light reaching the tank. I suggest you cover your tank in such a way that most of the day light is removed. Fed your fish sparsely. That means, only feed what they can and will eat within one minute. Get some algae-eating fish like a small pleco. Also change about 1/3 to 1/2 of your tank water to remove some of the algae floating in the water.

Too much light, whether natural or artificial (green/yellow LEDs can worsen the issue) can sometimes cause an increase in algae growth. Moving the light around a bit to see if it helps, changing to a different type of LED, or just cleaning the tank more regularly/adding in algae eaters may help.