Help w/ Alkalinity mystery (connected frag tank)

[drtechno]

Hi everyone

I have been completely stumped by this puzzle. I have no idea where to go from here, except to start dosing Alk in my connected frag tank (which makes no sense to me).
Here is the problem:

The alkalinity is consistently 0.5 dkh lower in my frag tank than in my display tank. Parameters (validated multiple times with 2 different test kits) are as follows:
Frag Tank
dkh 7.6
Ca 475
Mg 1450-1500
ph 8.3

Display Tank
dkh 8.1
Ca 475
Mg 1450-1500
ph 8.3

The alkalinity tracks 0.5 behind the frag tank consistently. If I raise DT alk to 8.4, frag tank alk goes to 7.9.

Setup
Display tank 200 gal
---AWC-waste--->
Frag tank 9 gal

AWC from the display tank (2 gallons per day) goes into the frag tank, where it overflows into the sink. I have verified that the outflow of the AWC parameters is exactly the same as in the display tank (as one would expect). Therefore, the 2 gal per day input into the frag tank is reflected in the DT parameters.
Evaporation in the frag tank is handled by 500ml RODI topoff.

Frag tank contains 14 small 1" acro frags.
Display tank is minimally stocked with small frags.

So why the difference in the alk readings?

1) If it is dilution effects from the rodi, I would expect the Ca and Mg to also show decreases as well?
2) If it is uptake by the small frags (I can't believe this would be true) then I would also expect to see decreases in Ca as well as.. in addition, for those small frags, I would expect 2 gal a day (~25%) daily water changes to replace any lost alk or ca faster than it is used.

In any case, I just do not understand the difference in the alkalinity between the two tanks..

 

[Randy Holmes-Farley]

You cannot detect the calcium and magnesium changes that would be associated with a 0.5 dKH alk consumption, which does not seem unreasonable to me given the scenario.

 

[drtechno]

Thx Randy

You cannot detect the calcium and magnesium changes that would be associated with a 0.5 dKH alk consumption, which does not seem unreasonable to me given the scenario.

Thx Randy
So you think the alkalinity difference is indeed because of the alkalinity consumption by the small frags (in a small tank)?
I don’t see any other explanation… although I would think that a 20% water change per day of “normal” alkalinity would be more than enough to keep up with the demand?

So is the verdict that I need to be dosing my frag tank in addition to using AWC waste water?