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Old 07-22-2014, 04:53 PM   #1
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Location: Cape Coral, Florida
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RO/DI System and chemistry questions

Our tapwater comes in about 370ppm TDS and 8.6 ph.

I filled my tank first with it (freshwater 45g tank), then struggled with the ph because even though I read all the "they adapt" that seems just too high.

So the LFS said use their RO/DI water for changes, and it will come down. Haven't done many changes but did one, and it didn't.

Tested theirs and it came in a bit over 8ph, but everyone says "RODI is so pure the tests don't work". So I think I'm good.

So finally bought a TDS meter, and their RODI water comes in at 350ppm. Quick check on meter against some distilled water (zero), and some RO-only water (13). So off to the LFS to ask them. The response was "our incoming water is so bad that's about right, unless you hit here on the first day after a water change".

So... I'm a novice at this (and that's being generous), but have an engineering background and this doesn't sound right, it just sounds broken. In fact it sounds like enough TDS is coming through to make me worry about things like chlorine. So I'm ignoring the LFS.

Bought a 5 stage RODI unit cheap at another LFS. Cheap as in no brand name and no labels, but (before you laugh) I'd done a lot of reading and it had a lot of things I wanted -- big, clear, easy to change filters, refillable DI with non-color-changing media that sits vertical, etc.

Hooked it all up and it didn't work at all... LOTS Of waste, not a drop of product. A bit more research says I needed a flow restrictor, so I used an included valve on the waste line and now I'm getting product.

RO output is about 30ppm (which looks high actually, but our water here is pretty bad), DI output is fully zero ppm, though very slow.

So ... long prolog, but two short questions....

Is there any guidance as to how much to restrict the flow? I seemed to get RO of about 30ppm regardless of whether I had it very restricted, or pretty open (huge difference in waste). Is more waste better (keeping the RO more clean?), or do I just adjust for minimum waste for adequate flow of product? Or (what I've done) is guessed at about a 4:1 ratio as that's what I hear most often?

And second question is water chemistry -- in a planted tank with fertilizers being added (Flourish comprehensive, iron, plus seachem root tabs under a couple places) do I need to dose RODI water with some remineralization, or is enough (and the right stuff) going to come from that?

I could also just blend with tap water (treated of course), but not any more sure if I'm getting the right solids. I'd rather add back if I need to.

PS. I do not know the water pressure here, my guess is about 50 psi or a bit better, but I do not have a gauge; I'll correct that soon.

PPS. The advice from where I bought this is "your water pressure is too low so you need to restrict as you did" and "it doesn't matter how much so long as the output is 17ppm or below". Well, it never hits that.
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