Q+A - Algae - fighting fire with fire

“I am told that I can fight bad algae by introducing good algae via a refugium/sump ... but how much do I need? For example, if I was to add cheato to my sump and light it in reverse to my display tank, would this work and how do I know how much cheato I need to buy? Thanks, Sean Richards, Somerset.”

Dear Sean

Reading between the lines I am going to make the assumption that you are asking a direct question about this particular form of algae control because you are struggling with keeping the amount of algae to an acceptable level.

‘Chaeto’ is an abreviation of ‘Chaetomorpha’ which is a form of filamentous algae that is used in marine aquariums (usually is a refugium). I notice you use the word ‘bad’ algae in your question – ‘Chaeto’ can become a problematic algae in the aquarium if it was not used in a separate filter itself.

You have hit the nail of the head with lighting the sump while the aquarium lights are off, while the display tank algae are not photosynthesising; the ‘Chaeto’ can begin stripping excess and unwanted nutrients from the aquarium water. The key to keeping ‘Chaeto’ working is to regularly prune it in order to remove the excess nutrients regularly and encourage it to grow.
As a bi-product it is also a good method to prevent pH fluctuations during the night.

In my line of work keeping algae to a minimum is essential for me to satisfy my clients, no one wants to see excessive algal growth when they walk into a home or business. This is why I feel you maybe better off simplifying the algal reduction techniques rather than trying to fight fire with fire.

1) Water test – Test for everything that algae assimilates i.e. different valencies of Iron, Nitrate, silica etc.
2) I would highly recommend Studying in books or reputable internet sites about ‘NPK ratio’ (Nitrogen, Phosphorus and potassium) if you can get this ratio correct in your system then the algae will be reduced greatly (don’t forget different ratios for different systems).
3) Any water quality issues – sort them out accordingly, if you have excess nutrients ask yourself why? Are you overfeeding?
4) It is sometimes easy to forget the simple things such as light, the first question I always ask when people have algae problems is – how long are your lights on for? Try Having a 1 hour ‘siesta’ period half way through the lights cycle. Adam Ewens sums this up simply in his 1976 paper: “The more light the more algae”.


Although algae and higher plants are both eukaryotic, algae is a lot simpler than higher plants and makes up the kingdom protista. It is because of this it is easily manipulated, the simple way to reduce algae problems in aquaria is to try to remove as much of its fundamental growth prerequisites as possible without unbalancing the necessary nutrients required by our corals and fish.
As far as ‘Chaeto’ is concerned I think you should only use it once you have tested for all algae causative factors. I feel that adding ‘Chaeto’ to a sump/refugium if you have an existing algae problem is possibly a ‘sweeping it under the carpet’ method!

Happy Fishkeeping

Ben Woodward