Adding Ro Water At Water Changes To Tap Water

[N0body Of The Goat]

I'm wondering if I could cycle my new tank with Southampton's finest, but then add 50 litres of RO each water change, thereby gradually lowering the pH and hardness of the tank water over time to nearer pH 7.0 over time?

I'm presuming that if I went in this direction, I would need to use a slow siphon technique to slowly add the RO to the tank.

I have a Nutrafin "master water kit" on the way, so I can carefully monitor most critical aspects of teh water condition, so at some point I would need to use some tap water so that the tank still had some buffering (or I could get the LFS to add tropical mineral mix to raise the RO water to pH 7.4).

Thankfully, from what I have read, most Zaire river fish do not need acidic water! (mainly Zaire river biotope of quirky fish planned)

 

[waterdrop]

Its a big decision to "go off of baseline" with your water. It should always be avoided if at all possible as you lose your "safety valve" of simple tap water being available for a large emergency water change. That said, if you decide to go down that path, there's been a lot written about it. Oldman47 has very hard water and an RO unit and is experienced with doing some tanks where the mineral content and pH are lowered by combining RO and tap. There are probably lots of writeups that could be found via searches on TFF.

I'm not sure how new you are to fishkeeping but if you are quite new then it sounds like you have chosen to try out a lot of challanging things all at once, with the big biotope tank and breeding and now the idea of altering your water away from baseline. This does get pretty far away from one of our principles here in the beginner section that its really good for new hobbyists to experience a first year or year and a half of fairly basic tank operation without advanced things being thrown in. This is because some of the "feel" of the hobby can only really be learned by actual doing and if one hasn't ever had the feel of a basic tank operating properly then of course it makes it harder to judge things that are going on in an advanced situation.

Of course rules are made to be broken, to some extent and I certainly don't want to put a slow-down on your good idea if you have the energy to carry it all out! Good luck, but do a lot of planning before heading in the altered water direction. You will also want extra test kits such as GH/KH kits. Tetratec and API make them as do others.

~~waterdrop~~

 

[OldMan47]

It is far safer to find out what the fish you keep really need and try to find some within the limits of your tap water than to try to move the water to accommodate the wrong fish. I keep mostly livebearers for a simple reason, livebearers in general find my tap water not only acceptable but ideal. Many egg layers need much less mineral content than I can easily provide. Because I have an RO in my home for drinking water, I find it relatively easy to provide lower mineral content water. That has led me to the folly of keeping some fish that require me to mix up a combination of tap water and RO.
You need to understand that most fish do not require a particular pH to be healthy and prosper. What we have done for the better part of a century as hobbyists is to measure pH and try to adjust it to particular values for our fish. The end result causes lots of stress for the fish. Most fish with a reputation for needing low pH really need water that is fairly low in mineral content. Such water often has a low pH in nature. Adding various acids, which I used to do, does not lower the mineral content but raises it. I never understood why my low pH fish were dying when they had the ideal pH in their tank until I found out that fish really need the right mineral content and can tolerate a rather wide pH value. Now I just let my "low pH" fish live with higher pH values in water that is closer to the mineral content that they need. The guiding parameter, short of buying electronic instruments, is GH as far as mineral content is concerned. The KH of water is more about having a stable pH than about mineral content.
I use both GH and TDS, from that electronic meter, to determine what my water is like and how to change it to suit some fish. I find that RO water or rain water can be treated as free of minerals, so I mix RO or rain with tap water to achieve lower mineral content water than pure tap water would give me. For my South American corydoras tank, I have been using a 3 to 1 mix. It is 3 parts rain or RO to 1 part tap water. That gives me a result of about 80 ppm of TDS but the pH is still around the 7.8 of my tap water. For many African rift lake fish, the ideal mix would be my tap water plus some added chemicals to make it even higher in mineral content. I have never studied the mineral content needs of river fish from Zaire so I cannot comment on them.