Need to lower GH: Hard Mode

[Baphijmm]

Tested the water in the Rio Negro tank this afternoon; it's been rather overdue, but as I'd expected, nitrites and nitrates are all quite good, near-perfectly balanced (zero nitrites, near-zero nitrates). pH is perfect (maybe a *little* high for the planned stock of the tank, but still pretty neutral around 6.8-7.0; once I can begin introducing tannin botanicals again, that should drop a little, so 👍). The one item that raised my eyebrows slightly was the GH; I expected it to be high for reasons I'll explain, but at >300 ppm, we've got a problem. I need to lower that.

This is absolutely a result of the water here; we're using well water (pros include no chlorine, cons include... well, this), and even though I fill this tank exclusively from an RO system, the calcium in this water is so thoroughly concentrated and suspended in solution, that's not nearly enough. High GH is a common problem throughout this state--you may have heard of "caliche", deposits of calcium carbonate that are extremely prevalent in the American southwest. (Ironically, in spite of this, the KH of the tank is actually quite low, ~20 ppm. Probably the RO system taking care of that, since carbonate is a much larger molecule.)

One might think the abundance of bladder snails in this tank might have something to do with it, but if anything that's just another symptom; there's enough calcium in the tank to support the likely literally hundreds of the little blighters in there.

So, my question is, how do I begin working toward lowering this GH? Hard mode: Since I already exclusively use RO water from our RO system, "changing water with RO water" likely isn't going to be a good solution. (Unless I feel like buying hundreds of gallons of RODI from the store every month.)

Is there anything... This is almost certainly a stupid question, but while I did ace freshman chemistry, I am absolutely not a chemist: Is there anything that might, for example, bind to the calcium ions in the water and... precipitate them out, or get them into big enough chunks that a water polisher might be able to just remove them? Thoughts?

[Jack.of.all.aquariums]

My first thought is something is wrong with your RO system. Have you tired replacing the cartridges/membranes in it?

 

[Baphijmm]

I believe we have, relatively recently (the system is not mine; it belongs to my employers), but I'll have to ask; I know the person who was supposed to have done it. I know that was a concern for them too, as I'm not sure anything had been changed on it in quite some time before.

[Baphijmm]

The person in fact could not remember, and looking at the system this evening, indeed the filters never got replaced; the last replacement on-record was in 2015. So, they're quite overdue! Will update if replacing the filters and cartridges helps; I'm sure it will. (I'd do it myself, but I've never actually worked on an RO system before! And since it isn't mine, I don't want to accidentally break anything.)

[Baphijmm]

Welp, after inspecting the system (and learning a lot about RO systems in general), yeah the filters were nasty. Indeed, it hadn't been serviced in seven years, and even that might've just been the carbon cartridge. The RO membrane canister was nearly sealed shut from calcium deposits inside it. So, after a thorough cleaning, replacing the filters, and flushing the system a couple times, the water coming out seems to be much, much better. We'll see over the next few days how well it pulls the GH down, but I think it'll do exactly what I need.

[Cinnebuns]

When you say "perfectly balanced" it makes me wonder if you are doing no water changes and top offs only. If that is true that's likely contributing.  Minerals do not evaporate so only topping off can cause your GH to go higher and higher with each top off. It sounds like the greater problem was the cartridge but this may be something to watch as well. 

Edited April 25, 2022 by Cinnebuns

[Baphijmm]

Nah, I've been doing proper water changes. Our water is just well water, and since the RO system wasn't functioning at all, I was just getting literal well water. We actually don't have a re-mineralizer; the reason the observatory has an RO system is because it was used in the past to help develop photographic plates, and the owners just got used to drinking water from it. We used to have a daytime groundskeeper who I suspect was keeping up with the unit's upkeep, but when he retired and the "new" guy took over (a former employee who'd come back), I suspect it wasn't on his radar.