Saltwater Aquarium Algae Control

by | May 15, 2005 | 0 comments

Algae Control Tips:

Here’s an strange sounding tip for reducing the time you spend cleaning algae from the glass: dose sodium silicate to promote a controlled population of diatoms. Diatoms! I know… bear with me.

Most aquarist’s introduction to diatoms is as an unsightly reddish-brown coating of their sand and rocks in the first few weeks of a new aquarium. At that point, most decide that diatoms are a “Bad Thing ™” and are forevermore willing to do anything at all to avoid diatoms or even the possibility of diatoms. When diatoms are out of balance (like that bloom in a new aquarium), they don’t do anyone much good. But when diatom populations are in a stable balance with other processes
in your tank, they are very good for the whole system.

Good things for aquarists might include:

  • Diatoms compete with blue-green algae for resources (#1 reason to mention this here).
  • Diatoms are much easier to remove from glass and acrylic and less unsightly than blue-green algae (light gold tint compared to algae’s green blotches).
  • Diatoms are part of “plankton” and just like plankton products that you can buy, they provide a healthy natural food for filter feeders.
  • Diatoms consume nitrates and phosphates from the water column and fix them into their tissue where it can be filtered out of your system via your protein skimmer (don’t worry, your skimmer won’t get all of them).
  • Diatoms on your sand are some of the best possible food for your cleanup crew and are likely to contribute to their longevity and increased health/diversity.

Now, how to get them to grow in balance in your aquarium. Diatom populations in home aquariums are largely limited by available silica. In order to get more diatoms without getting too many diatoms, you need to maintain a low but stable level of soluble silica in your tank water (I seem to get effective results from 1ppm, though I personally haven’t tried concentrations higher than 1.5ppm).

Before you dose something, you should know your tank needs an external supply, and you should be able to measure the level to know where you stand. Hatch makes a good kit for measuring soluble silica (detection level is .05ppm). To buy soluble silica, you want to buy the smallest amount of “water glass” you can buy from your local crafts store. “Water glass” is sodium silicate and it is used to preserve eggs (presumably artsy eggs, but I didn’t ask). The smallest I could buy from the local Michael’s was 1 quart, which is enough to keep an entire club’s tanks dosed for several years.

The stuff I bought was 41 baume, which is 29% silica by weight. I dilute this stuff into a quart of working solution so that each teaspoon of working solution will dose 10 gallons to a level of 1ppm. It takes 3 3/4 teaspoons of 41 baume solution to make a quart of working solution, which will treat almost 2000 gallons to 1ppm. (If you’ve got a tank under 55gal, this is probably too concentrated to be convenient, so you should probably add 1 1/4 tsp to make a 3x dilute solution and use three times as much when dosing). This stuff is very alkaline (even more so than sodium hydroxide — kalkwasser), so use gloves and clean up well.

Once you have your dosing solution mixed up, estimate your total water volume, which is probably somewhere between 66-80% of the tank’s total volume, depending on the density of your rockwork and the depth of your sand. Perhaps a first dose to 0.25ppm, so if your first silica test shows undetectable silica,
divide your volume of water by 40 to determine the number of teaspoons to add (divide by 13.333 if you made the “small tank” solution). Always dose into a high-flow area (remember the alkalinity). I suspect that anything up to 1ppm will be fully consumed within a week and you’ll probably be at the detection limit of the hatch kit (0.05ppm) within five days. I started out testing every other day and found that I have to dose about 0.33ppm soluble silica each day to maintain a tested level of 1ppm. Now I only test for silica every month or so along with my other water quality tests.

If you don’t use RO/DI filtered water, it’s possible that your water already contains silica (among other things). You need to take this silica into account when figuring out your dosing regimen, so having and using a test kit is doubly important for this case. Many people who find that they have trouble with diatom blooms are likely to find that their water has very high silica levels: 18ppm or higher. I recommend removing this from your water with an effective filter (RO/DI) to prevent blooms and then adding a precise (and small) amount of silica back to your tank to encourage balance.

As I said earlier, I maintain 1ppm in my main tank, and that seems to be the lowest amount that makes the glass scrapings much easier and the color of the dirty glass more pleasant (I’m lazy and refuse to scrape more often than weekly). I’ve heard of people having no blooms with levels up to 3ppm, but I do see a dusting of diatoms on my sand every few months (not a bloom and gone within 24 hours, the tiger tail especially loves diatoms) and am unwilling to raise the silica further without a clear reason to do so.

Ross (aka rabagley)

In any closed environment with animal life and light you are going to get plant life. Our tanks are no exception. So the only choice is whether you get the plant life you like like corraline algae, macro algae, sea grasses, mangroves, or corals. Or you get the plant life you don’t like such as hair algae and cyano.

Every bit of ammonia, nitrate, phosphate, toxin, heavy metal, and carbon dioxide consumed or bioaccumulated by plant life you like is one less bit to feed the plant life you don’t want or to adversly affect other life you like.

Sure use snails to consume the stuff you don’t, but real solution is to get plant life you like established and in control right from that start. That way you will have a tank with the plant life you desire.

Bob (aka beaslbob)

As long as the speed of phosphate removal is greater than the speed of production, all nuisance algae will eventually have to die off, as the phosphates are removed from the sytem.

The two main things that seem to confuse aquarists, are that:

  1. The resulting die-off of algae takes time
  2. Zero test levels from the water column is not the same as zero production, or uptake, of PO4

The aquarist needs to enable the system to dump phosphates BEFORE they can get used by algae; if 5ppm of PO4 is produced, and 5ppm uptaken by algae, one will still test a level of zero, as this only measures ‘excess’ that builds up in the water column.

The first, easiest way, to eliminate one major source, is via an RODI unit for water processing.

The second, is to use a phosphate sponge, like phosguard, or rowaphos.

It usually takes a combination of both, to achieve a ‘quicker’ result, though water changes,good skimming, etc., can achieve the same end,over a longer period of time.

The hobbyists also needs to understand that in addition to the daily production of PO4 by the life in the system, the phosphates introduced either via the source water, or livestock ‘surges’ (like when placing an amount of live rock , and it’s subsequent ‘die-off’ occurs in a system) need to also be removed.

Since algae require phosphates to grow and thrive, PO4 removal is not only the true root ‘cure’, it’s also the least complicated ‘treatment’ around, for dealing with nuisance algae.

Algicides just recycle the PO4 back into the system, to feed more algae.

‘Clean up crews’ (especially snails) merely recycle the algae, releasing the PO4 in their poop, to begin the process anew.


I have a little bit of everything:

  • I grow macro algae and harvest them.
  • I have some snails, hermit crabs, urchins, shrimp, stars…
  • I feed my tanks a little bit several time daily – they like food.
  • I keep my nitrogen sources as close to zero as possible.


I like the combination high in tank circulation and a good skimmer. The numbers dont mean all that much, but when you can see detritus, and snail droppings floating around the tank, instead of to the bottom, you are getting close. In addition a good cleanup crew will break larger particles of waste down, making it easier for them to be swept into the water column, and be removed by the skimmer.


Ask yourself how old your light bulbs are, and replace if needed.


Check your source water! You could be living downstream from a phosphorous mine and have no clue. Ask your local water district for the results of their most recent batch of water quality tests in your area before using tap water in an enclosed system.



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