I've been using ROWA for years. I recall sometime back reading that GFO can drop alkalinity. My last tank had a dosing unit and was heavily packed with fish and coral. My tank now is much less bioload, but due to some fish loss and overfeeding my phosphates went through the roof. I've since corrected with a larger amount of ROWA, but my alk keeps dropping since I've been doing it. I'm wondering if the two are related. My alk was usually around 8, but dropping down to 6.5. Just wondering what your thoughts were? Should I perhaps dose a certain amount of alk when I change out my ROWA? I don;t really measure it out, I just use like 3 heaping spoonfuls in my 60 gallon and change every 3 weeks now. Thanks for any input and help you can give me. Oh, I should mention that my mag and cal stay high at 1400 mag and 460 cal.
As with any Phosphate removal media ( especially high purity media ) over use of the media maybe problematic when bringing down high PO4 levels.
Phosphate works in a similar way to magnesium in that it can prevent calcium carbonate seed crystals from forming and precipitation cascade events taking place.
If your PO4 is very high and you try to counter this by overdosing Rowaphos to pull it out quickly you may encounter a sudden drop in Phosphate locally at the media surface which may set off a calcium carbonate cascade event.
You will then see a drop in Alkalinity off of the back of this, equally if your Mag is low this will also compound the issue. When this happens you may also see the Rowaphos start to clump as Calcium Carbonate starts to precipitate onto it.
You won’t see a massive drop in Ca when this happens and it may not even be noticeable as a drop in 1dkh Alk will only equate to a drop of around 8 ppm of Ca
To offset this use the correct amount of Rowaphos but change it more often until PO4 starts to reduce to manageable levels. I see you are only using 3 heap spoons of RowaPhos every 3 weeks and as such based on this I wouldn’t expect such a drop in Alk, it maybe that there has been a change
In bacterial action within the tank as this is an acidic process by nature and your drop in alkalinity maybe associated with that.
It maybe time to review your Calcium- Alk demand based on the current consumption of your corals if you are using a doser.
Best regards Tony