[Endlers]

Hello. I'm not a chemist by any means, just an aquarist who's in way over his head. I have a problem I need to sort out or I think I risk losing my fish. I've been trying to solve this problem myself all afternoon, and If you'll bear with me I'll walk you through it as briefly as I can manage. Any help in understanding what I'm dealing with or even advice on how to fix it going forward would be greatly appreciated.

Until yesterday I had very hard water, at least 300ppm Gh, 300ppm Kh, and 8Ph right out of the tap. I don't measure my TDS, but I know it's high obviously. I say at least, because the water maxes out every test I've ever tried on it, so it could well be much higher on all counts. My landlord has insisted on installing a water softener. My Ph has dropped to about 7.6, which isn't much of a problem. However my Gh has dropped to 0ppm, which is a death sentence for my fish if I continue changing their water with it. Kh seems to have dipped as well, down to around 200ppm.

To compound the problem, from what I understand, the brine solution in the softener removes the calcium and magnesium ions and exchanges each one for two sodium ions. If that's the case my TDS is going to shoot through the roof. That would put me to at least 600ppm sodium ions in my tap water, which from what I can gather isn't even safe for human consumption, much less my fish and aquatic plants.

Am I calculating this wrong or misunderstanding something I've read? Is it a simple matter of raising my Gh with calcium carbonate? I have crushed coral on hand for just that purpose, but I'm not sure it will matter based on my layman's understanding of the issue.

I suppose I should mention the landlord wont permit me to plumb a bypass onto the softener, I already asked about that.

[billnotgatez]

a side question

there are sold large bottles of water
could that suffice?

I see them in grocery stores and home depot

[Borek]

my Gh has dropped to 0ppm

If you use brine softener I would expect the water hardness to be very low at the beginning of every cycle, then gradually go up till next cycle starts.

Or at least as far as I understand that's how brine softeners work. No idea how long is the cycle.

[Endlers]

a side question

there are sold large bottles of water
could that suffice?

I see them in grocery stores and home depot

I'm afraid that could be my only real option, but it would be very taxing considering I run a small breeding operation. I'm also looking into getting an IBC tote or two and having bulk water deliveries trucked in. An RO/DI unit with a remineralizer is something I'm looking into, but from what I can tell that would require me to modify the plumbing as well, so at that point I might as well try to sneak a bypass valve onto the softener.
   

If you use brine softener I would expect the water hardness to be very low at the beginning of every cycle, then gradually go up till next cycle starts.

Or at least as far as I understand that's how brine softeners work. No idea how long is the cycle.

That is good to know.
Would you happen to know if the exchange rate I mentioned above is correct? I've found very little information about this online, and my only source is a post on a cichlid forum. To quote the original poster,
"Softened water has a higher TDS, (Total Dissolved Solids) than the water would prior to softening. The reason being that (if I am correctly informed) the exchange is actually 2 Na ions for 1 Calcium ion."

If OP is wrong it wouldn't be the first time a fishkeeper fudged some proper science, which is why I've come to the chemical forums.
I really appreciate both your responses by the way

[Borek]

Would you happen to know if the exchange rate I mentioned above is correct? I've found very little information about this online, and my only source is a post on a cichlid forum. To quote the original poster,
"Softened water has a higher TDS, (Total Dissolved Solids) than the water would prior to softening. The reason being that (if I am correctly informed) the exchange is actually 2 Na ions for 1 Calcium ion."

I have to admit I don't know the lingo used to describe water hardness in practical applications, I am much better versed in the real thing, for which GH, KH and TDS are just simple proxies - and after quick googling it is not entirely clear to me what their exact definitions are.

Ca being replaced by two Na sounds OK, what the resin does is it keeps the solution charge neutral, so for each Ca²⁺ removed it needs to substitute it with two Na⁺. Change in TDS should be not large though - you replace each 40 g of Ca with 46 g of Na (you need to scale it down the real mass of Ca involved, but the mass ratio is what it is). It would be definitely much worse for Mg (again, Mg²⁺ replaced with two Na⁺, this time it is 24 g with 46 g). But: TDS is not only these cations. it is whole salts, so cations and whatever counteranions were present, and the total change in TDS won't be as large as these numbers seem to suggest.

edit: to make things even worse, TDS is usually estimated from water conductivity, so it is a proxy on proxy