Low Nutrients Through 90% Waterchanges

DesktopReefer · Apr 6, 2022

Running a Pico tank is always interesting. As I’ve studied more and more, it seems that most corals do best with at least some nitrates and phosphates. Presumably they are helping build proteins and fuel growth. That being said, if you were to do consistent 90% waterchanges and feed very little, this should lead to nearly undetectable levels of nitrate and phosphates.

So here’s the question to the brainiacs of the room. Consistent huge waterchanges should theoretically be good because you are constantly replenishing all the elements of the water and almost starting fresh each time, but will the low nutrients from this process cause slower growth or less than ideal environment for corals?

blaxsun · Apr 6, 2022

You don't want 90% water changes, because you will rob the system of nutrients - and that will impact coral growth. There's a reason why people generally stick with 10-20% water changes every 1-2 weeks.

PatW · Apr 6, 2022

For a 90% water change, you would want to CLOSELY match the existing water parameters - salinity, alkalinity and temperature especially. It would work really well for restoring trace elements. But I would not recommend it.

If you phosphates are high, just using a little GFO (granular ferric oxide) should take it down. For a small tank, you are probably talking about less than a teaspoon unless your phosphates are high. You can put the GFO in a small mesh bag or coarse cloth bag and toss it into your filter sock.

I think you must be referring to SPS corals. LPS and soft corals are fine with fairly high levels of nutrients or so I understand. I have kept SPS and LPS but never softies.

I run low nutrients but they are measurable and I have an SPS dominant tank. I have tangs in my tank and they poop out the nori that they get every day which gives the corals something. I think most of the poop is taken up by the skimmer and nitrates and phosphates are reduced by chaeto in the refugium. I have a big system.

Also, if you have a bunch of flourishing SPS, they can take up over a DKH of alkalinity per day. It is just not practical to replace that with even daily 90% water changes. Dosing is a much better method.

If you want to have an aggressive water change policy, I would think no more than 50% normally and I would suggest something like 33% would be a better routine.