Stuck with high nitrates & algae :( Please HELP!

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Aquarium Advice Newbie
Jan 29, 2021
My aquarium sand is constantly developing brown/green patches, which I am assuming to be algae. Please see a quick video below.

The glass sides of my aquarium also seem to develop dark spots (algae?) and I don't know what else to do other than scraping it off but it comes back in a few weeks
Video -

When I check the water parameters, I do see nitrates are too high. I am using this product during water changes - "Seachem Prime Fresh and Saltwater Conditioner - Chemical Remover and Detoxifier", but doesn't seem to be helping a lot.

Please let me know what other steps I can take? This is a 55 Gallon tank with just two fishes right now. I have taken out the air bubbler as I read that more oxygen is detrimental to the anaerobic bacteria that processes nitrates. I am wondering if I need to replace the whole substrate or even the filter media? I have never replaced the filter media nor the sand and the tank is about 6 years old. Please let me know your thoughts. Thank you! :thanks:
The brown stuff is diatoms. These are common in new tanks and tend to go away on their own as a tank cycles and the nutrients balance out. But your tank is 6 years old that isnt the case. Diatoms typically feed on silicate and nitrate, and your nitrate is high.

Having said that, your diatom issue doesnt look that bad.

Too high nitrate is usually a sign that there is too much ammonia going into the tank, which your cycle is turning to nitrate. You arent overstocked, so look for other ammonia sources. Maybe overfeeding.

Maybe your tap water has nitrate in it. Have you tested your tap water or obtained a water report from your water company?

If your tap water is clear of nitrate, bigger more frequent water changes will bring down nitrate. Whats your normal water change schedule? How much, how often?

Test strips are notoriously not accurate, so you really have no idea what your water parameters are at. Get a proper liquid test kit or ask a fish store to test your water using one.

Seachem prime will do nothing for nitrate levels in your water. At best it detoxifies things a little for a day or so, but it is still there. There is actually zero evidence that prime even detoxifies nitrite or nitrate. Its a claim that seachem cant back up in any way, and their own FAQ on their website admits they have no evidence and have no idea how it does this.

Plants can be good nitrate consumers if they are compatible with your fish.

You have a misunderstanding on how anaerobic bacteria is grown and works. It true that anaerobic bacteria live in low oxygen environments and that they consume nitrate and produce nitrogen gas, but turning off the bubbler wont do anything to create a suitable environment and deliberately lowering the O2 in the water could be harming your fish.

In saltwater tanks these anaerobic bacteria can be supported in the live rock that are used to create those rock formations that are common in reef aquariums. The denitrifying bacteria that live on the outermost areas of the rock use the oxygen in the water in the process of consuming ammonia and producing nitrate. Then as the oxygen depleted water percolates through the innermost volume of the rock, that anaerobic bacteria can thrive and consume the nitrate expelling nitrogen gas. This is sometimes replicated in freshwater by having very deep substrate beds, like 6". Products like seachem matrix and biohome claim to be able to work in a similar way in your filtration if you have big filters and lots of media. Take a look at pond gurus youtube channel. But, ive never seen anything other than unverified claims by seachem or biohome on its effects on nitrate, ive never heard anyone actually say it works for them, and even if it does it takes years for anaerobic bacteria to develop. So its not a quick fix.

There are chemical media products that consume nitrate though. Nitrazorb for instance. But thats dealing with a symptom rather than a cause.


- Confirm your actual level of nitrate with a proper test.
- Confirm your tap water parameters with either a proper test or get a water quality report from your water company.
- Look for sources of ammonia production.
- Assuming there is nothing wrong with your tap water, increased water changes should lower nitrate. A 50% water change should reduce nitrate by half. Weekly 50% water changes should keep nitrate at low levels.
- Based on your test strip your nitrate is high and could do with being much lower.
- Lowering the nitrate should help with the diatoms. But its not that bad and if i could manually clean it up with weekly tank maintenance, id be happy doing that.
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Thank you @[Aiken Drum] for your detailed response :thanks: . I will look into these and get back!
You don't have any live plants in the tank so all the fish food and waste is going to make nitrate. If the tap water has nitrate then the lowest you can get your aquarium nitrate will be the same as the tap water nitrate. So check the tap water for ammonia, nitrite, nitrate and pH. Post the results in numbers here.

If the tap water does not have nitrates, then do a 75% water change and gravel clean the substrate every day until the level is 0pm. Then do a 75% water change and gravel clean once a week after that.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Having lots of floating plants in the tank can help lower nitrates. Some people have garden plants above the tank and let the plant roots grow into the water. These terrestrial plants use lots of nitrates and can help reduce the level in the tank.


The algae is growing because you have light and nutrients, and nothing in the tank to use the light and nutrients. If you don't have live aquatic plants in the tank, you only need the light on for a few hours in the evening when your home to watch the fish.

To get rid of the algae, either reduce the light and nutrients, or add a heap of live plants. Again, floating plants like Water Sprite and Duckweed are very good for this purpose. They use nutrients and will shade the tank so algae doesn't grow. And the severum will be able to eat the plants. Severums need a lot of plant matter in their diet and live plants like Duckweed are relished.


You don't want anaerobic bacteria in freshwater aquariums and there's usually too much oxygen in the water for them anyway. Having aeration maximises the oxygen level in the water and helps circulate the water around the aquarium.
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