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Will dropping CO2 levels always result in algae?

George Farmer

30 Jun 2007
Most of us are aware that stable CO2 levels are necessary to help prevent algae issues.

My question is can we drop CO2 levels without causing an algae bloom?

Here's my situation.

I have been injecting silly amounts of CO2, as I have no fauna in the aquarium. We're talking 50+ppm CO2, probably.

Now I want to stock some fish, obviously I need to reduce this to 30ppm CO2 maximum to be on the safe side. Ideally I'd like around 20ppm, as I've noticed most fish tend to be more active and eat better with CO2 levels below 30ppm. This is anecdotal evidence, but enough for me to believe it.

So am I likely to experience algae issues by dropping the CO2 by such a considerable amount?

Tank specs -

24w HO T5 over 60 litres.
Planting is around 20% footprint coverage of hairgrass.
Ferts - 5ml TPN+ and 5ml Liquid Carbon per day.
Filter - 1200lph external.
Water changes 2x 50% per week.

As I'm writing this I think I should be ok, with the combination of moderate-low lighting, decent filtration, liquid carbon, and water change schedule.

Thoughts welcome, especially from the likes of Clive, our resident Neo...
Hey George,
Neo spoke to The Oracle, who reckons that suddenly dropping the CO2 by half always presents a risk, mainly of the filamentous variety. This would be short term though. As long as the final value is sustainable for that lighting level the plants will make the adjustment after a few weeks. Typically, a change in Rubisco level has to be performed to compensate for the drop in CO2 availability. You're the type with over the top flow anyway, so I don't think it will be as severe as it might be for some others. The Liquid carbon helps as well. :wave:

I always inject silly amounts of CO2 in my new tanks, then drop it down prior to adding the fauna. It does trigger Spirogyra, but it is easily defeated. I think the initial high CO2 saves a lot of other potential algal issues, and is worth it.

George, I guess I just adjust photoperiod down at the start, dependent on plant mass and how fast they grow. My most recent tank was a concern due to the low plant mass, and E parvula being the only non slow grower in there. I started off with just a four to five hour photoperiod.

This tank, you may or may not remember, almost fell over on soft water containing no Mg, but would have most likely been OK otherwise. Personally, I just go along with the light intensity and ferts with which I intend to carry on with. All I do at the beginning is reduce photoperiod and hammer the CO2.

My tanks are typically EI, pressurised CO2 with T5 lighting, so I need to get things off to a good start. Especially as I seem to be using less and less plant mass at the start.


EDIT: Your latest tank on Flickr has seriously low plant mass, which will take a bit of nurturing. I`m sure you will get the required result. :thumbup: