Mixing RO/DI water with freshwater


New member
Hello guys,

Sorry for the topics, but I didn't want to go off-topic in my rock topic. Anyways, I currently have a 50 gallon rainwater container/barrel that I use only for preparing water, back from my discus days.

I also have a 50 gallon aquarium with freshwater, which I'm currently draining. I'm doing the same training with the mixing container.

I read on this amazing forum that it's recommended to first place the rocks, then the sand, followed by the water. A very logical idea. This means that I need to mix my RO/DI water in my container, to avoid exposing the live rock / rock to freshwater. (It used to be in someone else's tank until +/- 1 week ago and is thus a little bit alive.)

Now onto my actual question. Even when emptying both the tank and my container, there is still a little bit of freshwater left inside. I can flip over the container to almost get all out, but of course I cannot do that with my tank. Meaning that there will always be some water left that the pump cannot get out. Of course this water is just regular freshwater, treated with JBL Biotopol (a freshwater 'chemical' that binds/removes chlorine and coppers). Will this be too problematic to get slightly mixed with the RO/DI water or should it be fine?

If it's problematic, then I'm not sure what to do as I don't want to wait several weeks for everything to evaporate.

gone fishin

New member
IMO the little bit of freshwater will be insignificant that would be mixed. I would not worry about it. Good luck.


Reef Monster
Not a problem. I always have a little tap water somewhere and if you believe in your tank upkeep regimen and are fairly diligent, any that you do have will be removed. If worse comes to it, you can always add a little carbon or even some Amquel. I use it in my Freshwater ATO and it does the trick.


New member
I wouldn't worry at all.

I get a little dechorinated tap water in my tank all the time, when I clean equipment or change media in my reactor, and i use it to wash my filter socks. It's no big deal. I've even had tank sitters use it to top off when I had to go out of town for an emergency. Of course your water could be much worse than mine, but I doubt it could be bad enough to hurt your tank at that amount and still be drinkable.


New member
We have very clean wster in The Netherlands are our water utility companies don't use chlorines, but instead carbon filters and the like. Thanks to you guys I'm sure that it will be fine.