Fish deaths in macroalgae tank - pH swings?

George Farmer

Lifetime Members
Lifetime Member
Sep 6, 2006
Stamford, Lincs, UK
I've recently set up my first SW macroalgae tank.

125 litre (33 US gall.), T8 and LED lighting, 30x turnover, 25Kg cured live rock, about 10 species of algae, RO + D&D H2Ocean salt, 10% water change per week. No skimmer.

Everything has been doing great. Minimal nuisance algae, plenty of macroalgae growth, 'ideal' water parameters.

Recently I added 4 juvenile tank-bred Banggai cardinals. The tank had been set up for about 5 weeks.

They've been feeding great on a selection of frozen foods and look in top health. I'll try to get some photos up soon...

However, over the last 2 weeks I've been losing the fish, one after the other. All four dead now.

All parameters test fine, undetectable NH3/4 and NO2, NO3 very low (probably could have more!), SG 1.022, temp. 25C/77F.

O2 is around 10ppm at end of photoperiod, 8ppm at start.

The only thing I can think of is the pH. It's around pH 8 at end of photoperiod and 7.5 at start. Could the swing affect the fish in anyone's experience? Maybe I need more alkalinity?

I've numerous expert SW friends and they've no experience of macroalgae tanks used as a main display, only refugiums lit 24/7.

I've been given a common clownfish to keep the kids happy in the mean time and it's thriving right now.

I wonder if I just had some sensitive fish?

Thoughts most welcome, please!

Tom Barr

Staff member
Jan 23, 2005
Your pH should not be that low at the start, it might start at 8-8.2 and go up to 9 perhaps during the day...........
What is the KH?

You have very strong growth of macro algae, like plants, they have a high demand for Carbon, unlike FW plangts, this also means CaCO3, which they use structurally for making their rigid parts in many cases, some do not, but others use lots.

They can remove a lot of the CO3, as well as Ca++.

You have set up a pretty new tank, so the pH will move more than most tanks, you also have decent growth etc, and are only doing 10% weekly.

I'd suggest doing 50% weekly for awhile. It's a small tank.
So this is fairly practical, 15 gal a week, maybe try 10 gal a week.
Perhaps some rot is occurring from the new set up and causing a decline in the pH. Try 10 Gal a week. Once things get going and doing well, back off to 5 gal maybe and then see about dosing some Ca++ as CaCl2 and some baking soda for KH.

Like FW tanks, the marine systems do extremely well if you do large water changes. This applies to reefs and Macro tanks, then you do not need to dose Ca/KH or not as much, as critical or as often, same for many traces.......

For smaller systems, this is practical for a 300 Gal reef, maybe not, but then again, maybe so given the cost of the intense lighting, massive skimmers, refugiums, various current and flow rates............

A decent wet/dry filter, small decent pump, a decent wave pump/system, decent light, + good water changes ought to be okay.

Also, see some articles on Reefcentral's macro algae section.

Tom Barr