Large Mixed Reef - Alkalinity / Water Chemistry

mfraembs

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My new 330 Gal Display (500 gal total) build has been progressing nicely but I have an odd chemistry issue that I have been dealing with. Back in January I cycled the tank with Dr. Tim's for about 3 weeks and slowly started to add the fish into it. All the fish have been doing great and luckily I have had minimal issues with the ugly tank syndrome! About a month ago I felt I was ready to start adding some coral and began testing my calcium, alk, and mag. This is where things got interesting...... My initial test showed that both my Alk and mag was extremely low. My initial test was the following

Salinity - 35
PH - 8.2
Nitrate - 5 PPM
Alk - 5.8
Calc - 608 PPM
Mag - 1115

Seeing this I realized I needed to get my Mag up to reduce the amount of participate so I ordered some BRS Mag general adjustment and some Bi-Carb Alk to minimize the PH swing due to the amount I needed to dose. I did 8 qty 50 gal water changes every 2 days and started dosing the Bi-Carb and Mag to get back to ideal parameters. Over the course of about two weeks I got my Alk up to 8.5 Calc back down to around 450 PPM and Mag up to 1390 PPM and then knocked down the dosing to about 50% for a few days and then 90% a few days after.

I figured I got the chemistry back in check and figured maybe something funky happened with the water chemistry from the Dr. Tim's fishless cycle. Things stayed nice and level for about a week and put some tester coral in the tank and all seemed well. I ended up ordering a bunch of coral from WWC during there live sale on R2R and they seem to be doing great but have noticed my Alk has been on a steady decline, I once again went to my dosing regimen to keep my parameters in check.

With a water volume of 500 gallons my Alk should be staying consistent as I only have about 20 SPS frags such as Monti's, bird nest, etc., maybe about 10 LPS, and around 20 softies such as Zoa's, Mushrooms, etc.

My tank is an island tank and the only thing out of the ordinary is that my display is located in the middle of my living room and drains into a concrete pit that I coated in Pond Shield which is an epoxy type coating. I don't have any reason to assume this could be affecting the water chemistry as I have spoke with several other people with large builds that utilized this product in a similar manner. The water is pumped from the pit into my sump and then back to the display. I attached several photos so you can see what the heck I am talking about! LOL

All my coral seem to be doing just fine and have been in the display a little over a week now. I am just baffled to what is absorbing / participating my Alk. There is just no way these little frags are consuming this amount of Alk in 500 gallons of water?

My current parameters are the following:

Salinity - 35
PH - 8.23
Nitrate - 5 PPM
Alk - 7.62
Calc - 419
Mag - 1339

My initial test was also confirmed by multiple test kits and all kits tested pretty close to what my Trident is showing. Excuse the mess in the fish room, I still have soooo much to do with this tank! LOL My Alk dipped to 7.06 two days ago and is when I started to dose again. Salt I am using is also Red Sea Black Bucket and am dosing 200 ml / day of Alk Bi-Carb and 40ml of Calc.

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mfraembs

mfraembs

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Cement impacts water chemistry, but if the epoxy is intact that won’t matter.

Did you do something to massively boost mag and drop calcium? That doesn’t just happen in its own.
I have 4 coats of the pond shield epoxy so there should be no contact from water and concrete. I dosed BRS Mag "General adjustments" and BRS Bi-Carb so I didn't have a large PH swing. Plus I did 8 large 50 gal water changes to try to pick up any unknowns that were going on.
 
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