Help Troubleshooting 18 mo Old Tank

[Excelsior856]

Hi everyone, I'm looking for some suggestions as I've run out of ideas...

My tank (RSM C130 - 34 gallon) has been up for about 18 months. Overall everything has been very smooth since the beginning. I have a variety of coral and livestock as you'll see below but over the last 4-5 months I've been running into a persistent algae problem.

I religiously do bi-weekly 20% water changes and all water comes straight from a BRS 6 stage RODI that I changed filters on at 1 year mark. I run CPE and Purigen in my media rack and change them out every 3 months. Lights are stock for the tank and are changed every 6 months. About 8 weeks ago I added a BRS mini reactor to run straight GFO (and I've also tried a GFO/Carbon mix).

I have a Neptune Apex so everything is monitored and stays very consistent. Here are readings as of today:

temp 79.8 (controlled)
pH 8.09 (varies 7.9 - 8.2)

NO3 - 0 (maybe .1 - I measure with a Salifert low-end test)
kH 4 (this is a little low - my pH and kH have always run a little low and I've never dosed)
PO3 - 0 (API kit)
Calcium - 400

I run lights 7 hours/day

I recently tried blacking out the aquarium for 4 days (sheet wrapped around) and it still didn't make much of a difference in the algea. It seems like GHA but to me seems overly persistent? When I do WC's I suck as much as I can out but it comes back in a few days.

Livestock are currently:
royal gramma
coral beauty
Carmel Clown
varius snails in CUC
(I had a cleaner shrimp that disappeared a couple months back and a banggai cardinal that disappeared this week)

I feed frozen Reef Frenzy (about the size of a pea, once every two days).

I appreciate any and all help to get this thing cleaned up. Thank you!

 

[Therapy]

99 times out of 100 it is a Nitrate issue. Often times this reads minimal as the nitrates are being taken up by the aforementioned Algae. Giving you the erroneous measurement. Further do you employ a skimmer?
For starters, while you consider other remedies, I would start by doing that WC every week instead of bi-weekly.

I personally am not a fan of chemical remedies. Which is sort of a misnomer on my part.

 

[Antics]

I second increasing the water changes to weekly. Doesn't sound like you are over feeding. The lights are only on 7 hours but does your tank get direct sun light by chance? What kind of rock did you use? Was if live or a dead base rock? Perhaps it's a leeching issue. I'd raise the kH for sure though.

 

[chipmunkofdoom2]

What is the TDS of the water coming out of the RO/DI unit? Unfortunately, just using an RO/DI system isn't quite enough, the RO/DI water needs to be tested as well to make sure the water contains an acceptable level of dissolved solids.

My money is with Therapy, this is probably a nitrate issue. You're running GFO, so your phosphates should be under control (and maybe you can increase the amount of that just a tad to make sure). Even though nitrates are measuring zero, we can discount that as being misleading. If you're having nuisance algae issues, you have nutrients, even if the kits read zero. The nitrates (and maybe phosphates) are probably being used before they can register on a test kit.

Additionally, alkalinity of 4 is very very low. I'd devise a strategy to bring that up as soon as possible. Carbonate (alkalinity) levels are a big contributor to pH, and low alkalinity will give you low or unstable pH. Check the Mg as well, if that's too low you're going to have trouble controlling your alkalinity and calcium. I'd also check the chemistry of your newly mixed saltwater to make sure the levels aren't way off.