losing alkalinity

I have recently had an outbreak of cyano bacteria. I'm not exactly sure what has changed in my tank to have it start, but it's a PITA to get rid of. It resides primarily on the right side of my tank. My gyre pump on that side of the tank was recently cleaned, I thought maybe it was running slower than normal. My feedings have not changed in 3 years either.

About a month ago, I noticed my calcium was stupid low. So I did everything I needed to to get it back up to where I wanted it, 400. I have been able to maintain 400, so all is well there. My ph is at 8-0-8.1. My alkalinity is at high 7's. My water temp is about 80, as it has been for 3 years. No change there. My nitrates are at 0, or pretty darn close.

I have tested my salt water reserve that was made using Reef Crystals, and all the parameters in that mixture are where they should be.

When I did a water change in the DT, about a day or two after, I begin to test, again. All parameters are pretty much right on and are consistently held in place. Except for Alk. That Alk drops slowly daily. I dosed 100ml of soda ash (BRS) one day, go back to test it the next day, and it hasn't budged. Or if it has, it slightly drops. I already have my BRS dosing pump running for 20 or so minutes every hour for 7 hours, starting around midnight (after lights out). I have held that dosing time for about 2 years.

So my question is this. Could this cyano be sucking the alkalinity out of my tank?

I thought about removing a good portion of the sand, and replacing with new, but I'm not sure that would even help me.

What are things could cause my alkalinity to drop?

As for my corals, I have smaller type frags everywhere, 3-4". Only coral that is of decent size is a toadstool and a hammer. Some of the zoas and my duncan are upset. I can only assume it's the low alk.

I think I have laid out everything you'd need to know. If not, please ask, and I'd appreciate any help.

Shawn
 
Cyanobacteria are unlikely to consume that much alkalinity, but corals and coralline could do that fairly easily, according to my computations. I'm assuming by "100 ml of soda ash" you mean 100 ml of dissolved supplement, though.
 
That's not a particularly large daily dose, assuming about 80 g of actual water volume in the system. You might see some precipitation happening on heater surfaces or the like, though. Tanks vary enormously in their daily consumption.
 
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