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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 !
 

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75g, 40g, 20g
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FWIW, I am also doing water changes with RO water on my 75g heavily planted tank. Just doing straight RO water changes (without adding in anything) will eventually make your plants unhappy.
With that said, I personally do not like using Equilibrium - Adds way to much K (potassium) to the mix.
A much better option is adding in CASO4 (calcium), MGSO4 (magnesium), and a little K2CO3 (which adds back some dKH).
With your tap at Kh - 60 or 3.4 degrees KH that is not too bad. I remineralize to 1 dKH which keeps the pH rock steady.
With your tap at Gh - 125 or 7 degrees GH that again is not too bad. I remineralize to 22ppm CA, and 8ppm MG. This works out to almost 5dGH.

As you stated "The reason I am using the RODI is my fish tanks were not doing good with tap" - was this the plants not doing well or the fish? I ask as I live on the Mississippi River and the water here is high in CA and low in MG (gotta love limestone). Your tap water could be mismatched as well which is why going RO is so helpful.
 

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If you are using a RO or RODI, you should have a TDS meter (inexpensive pen type thing on-line). You could measure your tap water, and then your RO or RODI, and even your tank water.
 
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