Low alkalinity!

BitFix · Mar 7, 2020

My tank has been set up for about 8 months, ever since it was set-up alkalinity has always been lowish, but now its getting to the point where I'm getting seriously worried, even though everything else is very stable. It is a 25 gallon/100 Liter system. I'm surprised, since all of my coral have been growing decently, and new corals don't seen to show any signs of stress.

Parameters:
Alkalinity: 4.8 dkh (It was 6-ish a few months ago)
Calcium: 450 ppm (Has always been stable)
Magnesium: 1350 ppm (Has always been stable)
pH: 8.15 (Has always been stable, which confuses me, since low alkalinity should result in unstable pH, right?)
Temperature: 26.5 °C/ 80°F
Salinity: 1.025 SG
Nitrate: 1 ppm (I have been fighting a hair algae issue lately, that's why it's so low)
Phosphate: 0.03 ppm (I have been fighting a hair algae issue lately, that's why it's so low)
Ammonia and nitrites: 0 ppm

Stocking:
2 Ocellaris Clownfish
1 Bicolor Blenny
1 Orchid Dottyback
4 Hermits and 4 Trochus snails
2 Peppermint Shrimp
1 Emerald Crab
----------------------------------------
1 Colony of candy-cane coral
1 Frag of Candy-Cane coral
1 Frag of Duncan Coral
1 Small Colony of Pavona Cactus
1 Frag of Torch coral
A few colonies of zoanthus

Randy Holmes-Farley · Mar 7, 2020

You need to dose alkalinity. The expected calcium decline is too small to readily observe.

BitFix · Mar 7, 2020

Randy Holmes-Farley said:
You need to dose alkalinity. The expected calcium decline is too small to readily observe.

Is it normal for calcium and alkalinity to drop at such different rates? I tried dosing alkalinity, but it would spike then slowly lower again, so I thought low but consistent was better. Do you think it would be a better idea to continue dosing even though the corals are thriving, or at least growing and looking well?

Flippers4pups · Mar 7, 2020

BitFix said:
Is it normal for calcium and alkalinity to drop at such different rates? I tried dosing alkalinity, but it would spike then slowly lower again, so I thought low but consistent was better. Do you think it would be a better idea to continue dosing even though the corals are thriving, or at least growing and looking well?

Around 7 dkh is where you should be as a starting point. Corals can adapt to low alkalinity, but around 7 is better.

How often do you do water changes and what salt brand do you use?

Weekly water changes with a salt that has a dkh of 8 ish should help keep it up, but amount of coral and their growth can deplete it quickly.

Making alkalinity solution is easy and can be done with sodium bicarbonate (baking soda).

BitFix · Mar 8, 2020

Flippers4pups said:
Around 7 dkh is where you should be as a starting point. Corals can adapt to low alkalinity, but around 7 is better.

How often do you do water changes and what salt brand do you use?

Weekly water changes with a salt that has a dkh of 8 ish should help keep it up, but amount of coral and their growth can deplete it quickly.

Making alkalinity solution is easy and can be done with sodium bicarbonate (baking soda).

I use IO Reef Crystals, I usually do 15% weekly, but honestly I've been slaking recently. I will try to do them more often and dose Baking Soda.

Randy Holmes-Farley · Mar 8, 2020

BitFix said:
Is it normal for calcium and alkalinity to drop at such different rates? I tried dosing alkalinity, but it would spike then slowly lower again, so I thought low but consistent was better. Do you think it would be a better idea to continue dosing even though the corals are thriving, or at least growing and looking well?

yes. Expected drop from calcification is 2.8 dKH for each 18-20 ppm calcium. That could fit your scenario given relative uncertainty of calcium kits. Alk can also drop as nitrate rises.