2 part question

Jahalu · Jun 7, 2016

Is it necessary to always add the same amount of each? My Ca keeps dropping, but my KH is nice and stable on target. Can I increase just the Ca I dose?
My current parameters:
KH 8.3 dkh
Mg. 1290
Ca. 400. (Has been dropping steadily from 430 on 5/19)
Salinity 1.026
I just started using 2 part about a month ago. Right now I'm dosing 10ml each per day. My tank is 75g.

kateater · Jun 7, 2016

30ppm Calcium in two weeks isn't bad. Mine drops 20ppm every two days and my alk stays pretty stable only dropping 1dkh every 5 days. Your mag seems to be a little low, I try to maintain around 1340ppm. But to answer your question, yes you can increase just the calcium.

Randy Holmes-Farley · Jun 7, 2016

It is not "necessary' to add both in proportion, but unless water changes are messing with the numbers, calcium is very likely not declining more than the proportionate amount of alk. Additions of alk that you are unaware of might cause this, and certainly water changes can, but there is no real way for calcium to decline in the tank without alk declining.

Jahalu · Jun 7, 2016

Ah, I think I understand. Perhaps it is just a scale difference in the way we measure each? ie a 30ppm drop in Ca may not correspond to any noticeable change in dKH because they are measured on different scales. Has anyone calculated how much Ca needs to change to correspond to a change in dKH? (Assuming both are being consumed proportionally.) Hope this makes sense.

Randy Holmes-Farley · Jun 8, 2016

Jahalu said:
Ah, I think I understand. Perhaps it is just a scale difference in the way we measure each? ie a 30ppm drop in Ca may not correspond to any noticeable change in dKH because they are measured on different scales. Has anyone calculated how much Ca needs to change to correspond to a change in dKH? (Assuming both are being consumed proportionally.) Hope this makes sense.

When calcium carbonate forms in seawater, it consumes 18-20 ppm calcium for each 1 meq/L (2.8 dKH) of alkalinity. A two part is designed to match that.

Seawater has way more calcium than alkalinity in it, so that drop mentioned above is a 40% drop in alkalinity, but only a 4% drop in calcium.

Thus, you often do not even notice the calcium change for a long time, even when it is proportional to a detectable alk drop.

Jahalu · Jun 8, 2016

Thank you for the info. I actually have the opposite happening where my alk is stable and Ca is dropping. I'm now thinking it may be because last month, when I began dosing, my alk was very low but Ca high (which goes along with your explanation) and I started to dose more alk than Ca. I'm now thinking I may have caused an imbalance. If I continue to dose equal proportions now, should it balance out eventually? Or should I increase Ca for a few weeks to offset the extra alk I've been adding over the past few weeks? How would you approach this now? Hindsight is 20/20 - I really should have dosed equal proportions from the beginning.

Randy Holmes-Farley · Jun 8, 2016

Jahalu said:
Thank you for the info. I actually have the opposite happening where my alk is stable and Ca is dropping. I'm now thinking it may be because last month, when I began dosing, my alk was very low but Ca high (which goes along with your explanation) and I started to dose more alk than Ca. I'm now thinking I may have caused an imbalance. If I continue to dose equal proportions now, should it balance out eventually? Or should I increase Ca for a few weeks to offset the extra alk I've been adding over the past few weeks? How would you approach this now? Hindsight is 20/20 - I really should have dosed equal proportions from the beginning.

If you dose equal amounts of a properly made two part, in enough quantity to maintain alkalinity, calcium will stay about where it is (regardless of the actual value) unless messed with by a water change. Over the long term, smaller effects may become noticeable, but not in a week or two.