KH issues with Planted Tank with Discus

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Hey everyone! I having been battling with myself on what is the correct thing to do. I have. 125 gallon discus planted tank with some other fish that go well with discus in it. Also have a 40 gallon goldfish tank. I have been using strait RODI water in it. The first fill was with strait tap water. My water parameters in my discus tank are



Ph - 7.6
Ammonia -0
Nitrite -0
Nitrate -0
Gh -125
Kh -60
High ph -7.4


I have been doing weekly water changes of 23 gallons a week. And the water parameters have not changed other than my KH is becoming lower but not my GH or PH. I understand that my GH and KH should be softer for discus than most. I understand that I should be adding Minerals back into the water before adding strait RODI water in my tanks correct? My concern is that if I do that using a product like Equilibrium that it will make my GH go up. I know that I can use an alkaline buffer to help with my KH but won't that make my PH go up? I am concerned that my KH will continue to drop causing an issue with my PH being unstable. Do I add Acid buffer in the that alkaline buffer to make the ph go to where I want it? I am so lost on what to do. I feel that i may be overthinking all this. There are plants in there so I don’t want my fish or plants to die by using water changes with RODI water. Again i have not been re-mineralizing with anything yet I understand that to be bad. My ideal with by using acid buffer, alkaline buffer, and equilibrium in the RODI water, I can create the perfect water for my plants and discus. Should I do that or just cut tap water with it and then use Prime?. Advice is so needed. I am lost on what to do. I do not want to harm my plants or fish by using the RODI. The reason I am using the RODI is my fish tanks were not doing good with tap and a friend told me RODI is the way to go and so far it has worked great. Everything has been going smooth so far and they seem happy but now my KH value being come low every week is starting to worry me. I am lost on how my PH is still high but my KH is low? How does that work. What is the right thing to do? Thanks so much everyone !

 

[Allwissend]

I am not a discus expert but had them in both types of water. Issue with harder water is more with breeding as is the case with many SA Cichlids , but can't imagine softer water being an issue in any case. Just to say there is no magic number that needs to be achieved or else...

I would suggest pursue a good desired GH and KH first, pH will come afterwards. A GH of 5-6°dH is a good general target, lower it if you want. For plants I would strive to have at least 3mg/L Mg in the water. With a discus bioload 3 °dKH is a good minimum to have. The reason for that KH is not pH itself but having enough bicarbonates to allow ammonia conversion between water changes.

pH will likely stay at 7-7.4 if you have a decent KH and not a lot of acids in the tank. The good part is that when the KH is lower it's easier to lower the pH. Tannins and other organic acids will slowly lower the pH. CO2 injection can help plants and also lower the pH... this would be my first choice. Faster plant growth and low pH in one package It doesn't have to be 30ppm CO2, just moderate values if you want it. These are some ways to reduce pH but to reiterate i would not worry that much about pH, at least not in the 6-7.5 range.