Considering Switching To Ro Water


New Member
Jan 10, 2021
Hi everyone,

I’m considering switching over to RO water, and have a few questions I’m hoping you can all answer. A few things that I feel will be relevant:

  • I change about 75 gallons of water per week in a 125g tank. This would leave 50 gallons of water left, but a lot of that is taken up by substrate and hardscape. I genuinely have no idea how much water that displaces, so an accurate percentage of water changed is hard to give.
  • I have about 180 lbs of Seiryu stone in my tank, along with pretty high CO2. This leeches a ton of Ca and Kh.

Here are my questions:

If I go forward with RO water, I’ll need more than 55 gallons of water storage. I’ve found a tall, narrow storage tank that will hold 80 gallons of water, but the top is somewhat closed and there is a bulkhead near the bottom to connect a pump. I know that I should remineralize before pumping the water back to the tank, but my problem is that I’ll never be able to pump 100% of the storage tank out. After the first water change, how would I go about figuring out how much Ca and Mg to add to the new RO water? Would I need to calculate how much water was pumped, and change the amount of Ca and Mg I add each and every time? This feels both tedious, and prone to miscalculation.

Second question… With the rocks that I have leeching a ton of Ca, the water hardness rises a not-insignificant amount every week. When I do my water change, there is going to be a huge drop in hardness. Could this dangerous to the fauna? I don’t know exactly how much Gh and Kh leeches during the week, so I’m not really able to give exact numbers. What I do know is that my TDS currently goes up about 100 points during the week. This obviously includes fertilizers and dissolved organics, but a lot of it is hardness and Kh.

If I make the change to RO water, will the rocks leech even more hardness than they currently do, or will that not make a big difference?


Apr 16, 2018
I would worry about remineralize the RO/Di water. The stones should take care of that.
I use RO/Di water in my ADA 120p and all my other aquariums. It my shrimp tanks I an salty shrimp but in the 120p with rainbows and plants. I keep the water soft for the plants.
I also use phosphate removal media in my ADA es1200.
Using both keeps my phosphate levels ultra low and there by stoping hair algae and all other forms of algae in the Aqurium. Been doing this in my aquariums for 30 years now.