Nitrates were 80 so did bigger 2/3 change today with 40l RO. 200 litre tank. Can I do another water change soon or with the shift in parameters today should I leave it a bit? How long should I wait? See parameters below. Want to get nitrate down further and soften the water a bit more, but not below GH of 11.

I haven’t used RO before but wanted to start, so figured that with the tap nitrates at 30 ish it’d be good to start now. What I didn’t consider was the need to do more water changes in a shorter space of time could potentially shift the parameters too quickly for the fish. My plan is to use 1/3 RO with each water change.

Liquid test results:

NTL Ammonia 0
NTL Nitrite 0
Salifert Nitrate 5-10, API 30
Ph 7.4 API
GH 13 NTL
KH 10 NTL

Tap water
GH 17
KH 12
Ph 8 (after 2 days in a glass)

Thanks

If you remineralise the RO you can do bigger changes as you can control the gH and kH to not cause too big a reduction in parameters. Otherwise, you can do a water change with tap and RO mixed, but the reduction in nitrate will be restricted by whatever nitrate your supply has. If you're aiming to also reduce gH and kH you can use neat RO and do a small change daily until parameters in the tank are where you want them, then ongoing, use a suitably balanced mix of RO and tap, or remineralised RO.

Thanks but these are not options. Remineralised RO will cost £13.50 a week plus petrol as nowhere local has it. Plus no space to store it even I did buy all those extra canisters, and the canisters also cost a fortune.

My question is how long until I can do another significant water change with RO? Due to parameter shifts. Or can I calculate how to keep the parameters the same as now and whether that will reduce or increase nitrates? Wouldn’t know where to start with maths.

I don’t have time for daily changes, I live alone and work a lot of hours, it’d be dangerous to mess with the fish tank dog tired. Tried it before and almost killed all my fish!

Thanks

If you're using neat RO then the maths is simple. Whatever percentage change with RO you make is the same percentage reduction you'll get in all parameters (except pH). So, for example, a 33% change with RO would reduce your kH to approximately 6, gH to approx 9, and nitrate to 20ppm. IMO that's too much of a drop in one go so I'd restrict to 15 or 20% and do a change every other day until you're where you want to be.

It's an awful lot less expensive to get your own RO unit and remineralise than to buy it in. I use all RO and remineralise as I have high nitrate in my tap water and keep fish with varying parameter needs and it takes about 500 litres a week to do a 33% change on all of my tanks. You can get an inexpensive RO unit for around £40. Plumbing it in is simple (I did it myself)and minerals are not terribly expensive to buy.

May I suggest another option that did work for me?

My tank/s always had high level of Nitrates (~80ppm) so algae was always a hidden threat. I bought some well developed floating plants and threw them in, tested the water a week later to see 12.5ppm (Tetra) and 5pp (API). Am happy.