Macroalgae causing alkalinity depletion

[Maivortex]

I have atank with nothing but macroalgae in it. Recently the alkalinity has dropped to 80ppm. The tap water used to fill the tank in 150ppm. I dont add supplements. What could have consumed all the alkalinity?

[bertoni]

Some macroalgae consume alkalinity and calcium. Coralline will consume a lot, too. What exactly is in this system?

[Percula9]

Halimeda will consume calcium and alk.

[Randy Holmes-Farley]

Only calcifying macroalgae deplete alkalinity. Very few macroalgae species (like Halimeda) calcify.

Coralline algae and many other small calcifying organisms on live rock will use a lot of alkalinity, and may be the biggest user in many tanks.

Rising nitrate levels will also cause alkalinity depletion due to the proccesses involved in the nitrogen cycle.

[Percula9]

Halimeda Calcareous Algae
Further to the messages on _Halimeda_ that are contained in the Message/Mailing list archive, I would like to add the following:

Halimeda species are calcareous algae, meaning they will uptake calcium from the water and use it to grow larger although they never really get real large and will certainly not become the type of algae you would want to remove from your aquarium. They are real nice additions and I encourage everyone to try and add some to their reef tanks.

Two common varieties found are H. discoidea and H. opuntia. The former looks like an open piece of lettuce and is thus ribbed. The latter is characterised by leaflike extensions that grown from the main branch.

Both are usually found on sandy bottoms but H. discoidea can attach to hard bottoms such as rock too. Make sure you provide the rigth attachment means when you place them in your aquarium. Sand is best in both cases.

Halimeda grows well in environments with high calcium, medium light, availability of low amounts of PO4 (real low ones suffice) and low amounts of iron in the 0.05 ppm ranges. You may need to supplement the tank with iron if you wish them to grow better and be greener. You can read up on iron supplements in this SW library under TAT Product Related Articles.

Albert

[Randy Holmes-Farley]

Did you think I had said you were wrong (I didn't), or were you just expanding on your answer?

[Percula9]

I read your answer to fast and misinterpreted. I apologize, plus I wanted to expand my previous post to include a better answer.

[hilgert]

Randy, does cheato use calcium? It was my understanding that it requires calcium to some extent, but I cannot remember where I heard that (so it must be true :) ).

-Hilgert

[bertoni]

It won't consume any significant amount, even if harvested. If it's not harvested, it won't remove any.

[Randy Holmes-Farley]

I agree. Chaetomorpha uses little to no alkalinity, and the calcium amount is likely trivially small. Maybe less calcium export than removing a similar mass of tank water to the amount of plant matter removed. :)

[mick243]

dont know if it happens in marine plants, but some fresh water plants will use carbonate ions and bicarbonate ions as a carbon dioxide source if (and this is unlikeley in most home aquaria) carbon dioxide is becoming limited.

[bertoni]

Saltwater plants will take up carbonate and bicarbonate as well, but the alkalinity is released after photosynthesis consumes the carbon, roughly speaking.

[Randy Holmes-Farley]

Yes, algae that use bicarbonate and carbonate for photosynthesis (many do) necessarily release the alkalinity that they took up as they have no way to further use it:

HCO3- ---> CO2 (used by the algae) + OH- (released by the algae, so releases the alkalinity)