Water Hardness Totally Confused

With the API tests, the number of drops for both GH and KH give the value in dH. So your GH is 11 dH and your KH is 12 dH. The conversion factor of 17.9 is for both GH and KH to convert from dH to ppm.
GH and KH ranges in fish profiles are given in either dH or ppm and your 11 dH  for GH converts to 197 ppm and 12 dH for KH converts to 215 ppm.


GH is the most important parameter for fish. KH is only important to stabilise pH so doesn't impact fish directly. pH is also important but it can be outside a species' range provided GH is well within it.


The table in the instructions is a rather inaccurate list of fish they think can be kept at certain hardness levels (inaccurate as, for example, they say livebearers need 100 to 200 ppm but they actually need over 200 ppm with some species needing over 250 ppm; and the table might lead some to think that marine fish just need hard water not salt water!) Ignore this table, and look up fish on a reliable website to see what the hardness range for any species is. Fish should be kept in water with a hardness around the middle of their range.


Your tap water hardness is too hard for those species which need very soft water but too soft for hard water fish. However there are species which will do fine at your hardness.
But if you want to keep fish which need softer water than your tap water, you will need to mix your tap water with RO. The amount will depend on which fish.
It is possible that using RO to reduce GH will have no effect on pH; ignore the pH level and go by GH.

If you tell us which fish you want to keep, we can tell you if you need to use RO water or whether they would be OK in your tap water.

Words are not a good way to look at hardness. My tap water is classed by my water company is 'slightly hard' but at GH 5 dH, it is actually soft in fish keeping terms. This is why we always ask for numbers rather than words when we ask for hardness.


Cherry barb - 2 to 20 dH (36 to 357 ppm)
Dwarf neon rainbow - 5 to 15 dH (90 to 268 ppm)
Penguin tetra - up to 20 dH (358 ppm)
Cardinal tetra 1 to 12 dH (18 to 215 ppm) with wild caught fish needing nearer the lower end
Black widow tetra 5 to 20 dH (90 to 358 ppm)


The only one to be careful of is cardinal tetras; even commercially bred fish have your tap water hardness right at the top of their range and fish should be kept with GH near the middle of their range. But mixing three quarters tap with one quarter RO would reduce your hardness to around 8 to 9 dH which would be better.

Make up the tap/RO mix at the level you want to aim for, then do 25% water changes every few days. Once the tank water tests at the same GH as the mixture, you can do weekly 50%+ water changes with the mix.

Don't forget that you won't be able to do water changes using all tap water, so make sure there is always some RO available for emergency water changes (eg sick fish, something gets in the tank that shouldn't be there etc)

You want to reduce the GH by 4 dH at most.

If you do a 25% water change using water at 7 dH instead of 11 dH, it will drop the hardness by 1 dH. (Those numbers just make it easier to do the calculation  if you use 8 or 9 dH it will drop the tank hardness less than that)

ok so a retest now I know what I'm doing gave me a GH of  drops so 232ppm if I multiple by 17.9.  Now I'm confident of this.

I also tested my newly acquired water from spotlesswater.co.uk and it was pale green first drop so as expected nothing in the water.

This is where my calculations get a bit hazy haha.  I added, 12 litres of spotless water, and 24 litres of my tap water which goes through the Pozzani filter to remove very high nitrate.

Tested this mix and it was 11 drops.  I was expecting 9.  So it's decreased but not by quite as much as I had thought.  Not entirely sure what has happened there, is it possible the water didnt mix although even I can't imagine how that could happen.

I can test my tank, 165 litres so now has 36 litres of my mix in it.  Think that means by my calculations, it was a 22% water change (rounding up) of which 33% of that 22% was RO so I should read 7% drop of 13 which means I might see green on the test at drop 12 but it could easily remain at 13 haha. 

ok so it's not an exact science that 50/50 RO and Tap will result in a 50% drop? 

The tank has now dropped 1GH as expected and attempted.

Hi @Marcus_F

The only thing I would add to the instructions by @Sue is to keep an eye on KH to ensure that it doesn't drop below 4dKH. I hope @Sue doesn't mind my mentioning this. If KH falls below 4dKH, there is an increasing risk of what is known as a 'pH crash'. At this point, the pH can fall to well under 7.0 and this can affect both fish and nitrifying bacteria. It is advisable to maintain pH at no less than 6.0. pH crashes do happen - I know from my experience.

JPC

Thanks for the reminder. I know from personal experience as well.

Since GH and KH are virtually the same, if GH is above 4 ppm, KH should also be above 4 ppm, but it is worth checking to make sure.

Hi @Sue

I suspect you meant to write 4dGH and 4dKH, not ppm?

JPC

Thanks, read about the PH Crash and KH above 4.0.  Would the KH drop by 9KH if the GH has only dropped 3?

Am testing it all, not sure how soon after water changes I can test GH, KH and PH of tank?  I’ve been leaving it 6hours to settle down.

Thanks @jaypeecee and @Sue I think i'm getting there.

Today's tank reading GH 11 and KH 9, I'm aiming for GH of 9 and it's dropping 1GH a change when I do 25% so it's getting there.  Will start to pay careful attention to KH as that has dropped one more than GH.

Tested my PH and I cant read the colour.  Firstly I thought I could get away with normal PH test and it came out at 7.6 but in then testing using high range which I've had to do prior to my water work it's not really any of the colours so I'm not sure.

Posted a pic, what do you reckon is the closest reading?

Untitled by Marcus Fellerman, on Flickr