Alkalinity maintenance with kalkwasser

[nero0762]

I'm having trouble maintaining alkalinity, and I'm looking for some help/suggestions...

First, for context our setup is:

• RedSea Reefer 425XL (88gal display & 24 gal sump, ~100gal of actual water)
• Its been up and running for about 5 months.
• It currently contains (see pic):
  • 2 ocellaris clown fish
  • 1 royal gramma
  • 1 peppermint shrimp
  • 2 skunk cleaner shrimp
  • 1 bloodfire shrimp
  • 1 purple tang
  • 1 potters angel
  • 1 condy anemone
  • 1 rainbow bubble-tip anemone
  • 1 brittle star
  • misc snails and crabs (CUC)
  • 2 small clumps of waving hand xenia
  • 1 hammer coral frag
  • 2 tiny polyp frags (1 polyp each)
  • 1 medium polyp frag
  • 1 small torch coral
  • 1 lavender hairy mushroom
  • 1 small maze brain coral
  • ~70 lbs of Pukani rock
• DIY ATO system that adds 1-2 cups of water at a time, as needed

Most of the coral is relatively new to the tank, with the polyps and the hammer coral being the "oldest", and added about 10 weeks ago.
Around that time we started a very slow drip of kalkwasser mixed as potently as possible (2 teaspoons/gal). This would add about 1/2 gallon of fully saturated kalkwasser over about 5-6 days. Before starting the kalkwasser I adjusted the magnesium, calcium, and alkalinity using the BRS bulk kalkwasser starter package (kalkwasser, calcium chloride, sodium bicarbonate, magnesium mix).
The calcium levels decreased very slowly (~5ppm/day), but alkalinity dropped ~1.0 dKH per day. I've been adjusting the alkalinity manually with the sodium bicarbonate solution every day or two. As we added more coral I decided to switch to adding the kalkwasser directly to the ATO. I started with 1 teaspoon/gallon. Again, the calcium levels dropped very slowly (~2ppm/day), but the alkalinity continued to drop around 1.0 dKH per day. Even after upping the potency of the ATO kalkwasser to 2 teaspoons/gal, the alkalinity still drops ~0.75 dKH/day
pH is varies between 8.0 and 8.20 pretty closely following the lighting schedule. That's up about 0.1 since I switch to kalkwasser in the ATO.

So, what are we doing wrong? I was under the impression that kalkwasser would be "the" answer for a tank with a low/medium load, providing stable calcium and alkalinity. The tank has a few different types of coral, but they're all pretty small, so I can't imagine the load is very high. And the calcium levels are dropping, but just a bit. But the alkalinity falls off pretty quickly.

Suggestions?

 

[ReeferMaddness843]

Are you testing at same time each test? Alk will fluctuate throughout the day as its consumed.
Idk but following. I use a 20 gal mix of kalk in my ato container at 1tsp per gallon. Run an 80 gallon with a 20 long sump. Roughly 65 gallons total water volume. Lots of steady growth in sps and LPS. Pretty heavily stocked with coral frags but light bioload with fish. I use the kalk+2 (brightwell I believe, will check in a moment). It handles my Alk, calc, and mag. (Some disagree with the solution having enough mag to accommodate its use, but I have seen no drop) That may be due to the fact of weekly to biweekly 5 gallon water changes dependent on tests keeping mag stable though. I use Fritz RPM salt.
Alk 8.6
Mag 1470-1500
Calc 470-480
NO3 5
Po4 0 (need a better test kit for low range)
1.026