How Much Ro Water Is Too Much

[Rexenator]

I plan on adding a school of cardinal tetra to my 55 gallon planted tank. Everywhere I've been shows that they require soft water, low KH, to thrive. Our water here is ridiculously hard. I just tested it with an API KH kit and it takes 19 drops to change the color from blue to yellow. The chart with the kit stops at 12, and each drop corresponds to one degree of hardness. I mixed up different ratios of RO and tap water, and tested each. These are the results:

0% RO: 19 deg
50% RO: 12 deg
75% RO: 7 deg
87.5% RO: 4 deg
100% RO: 2 deg

Cardinal tetra require 2-6 deg KH, so this puts me in the 75-87.5% RO water range. I'm not worried about cost, and my system will produce 75 gallons per day so that isn't an issue. Is there any issue with using this amount of RO with my tank?

 

[HAMBONE]

Ahhhh the question that fish keepers has been asking for years..

I'd say no, but i'm no expert..

 

[TwoTankAmin]

There are no KH requirement for fish. What there are is conductivity or TDS requirements.
 

 
KH doesn't affect fish directly, so there is no need to match fish species to a particular KH.

http/fins.actwin.com/aquariafaq.html
 
But that is only part of it. There are wild cardinal tetras and there are farmed cardinal tetras. Wild ones are imported and would need acid water to start. Farmed can do fine at higher. pH matters here as well.
 
Next,
 

 
Temperature: 20 – 29 °C
pH: 3.0 – 7.5 although wild specimens do best in more acidic conditions.
Hardness: 18 – 215 ppm; towards the lower end of this range for wild fish or breeding.

from http/www.seriouslyfish.com/species/paracheirodon-axelrodi/
 
you need a TDS meter and a pH meter. You need more if you want to mess with water parameters beyond about 50/50 tap/ro.
 
Yes there is likely a problem with using more than 50/50. You need to learn a bunch more about water chemistry.

 

[Rexenator]

I've a pH meter here, TDS meter is on the way.  I'll take a look at my dissolved solids once it gets here.  My pH here is consistently 8.3, which is typically too high for these fish.  I was hoping to lower the KH enough that the water would have fewer buffering agents, so the driftwood in my tank could help to lower the pH without me having to add peat.

 

[TwoTankAmin]

I have pH 7.1 and TDS of between 60 and 83 ppm depending on recent rainfall. I keep a few wild altum angels which I keep in a blackwater environment.. They were brought into pH 4.2, TDS 30 ppm water and over a number of month I moved them up to 6.0 and 60s TDS with a KH of about 4 dg. The tank tends to rise towards 6.5. Here is what I use to hold the tank in the 6 - 6.5 range:
 
50-60% ro/di water
catappa leaves (changed every other week)
alder cones and peat in one filter (replaced as needed)
muriatic acid (I have to add this with water changes and usually once or twice in between to hold the pH closer to 6 then 6.5. Muriatic acid raises TDS.)
 
In addition to help with the water params I add two things that do not lower pH, rooibos tea and Kent Black Water Expert. The rooibos actually raises the pH by about .1 but has so many benefits it is worth it.
 
I tend to hold the TDS in the low 60s
 
I run a continuous monitor on the tank for conductivity/TDS, temp and pH.