Increasing TDS readings.


First time for me using RO water. To start with I was getting 0 TDS readings straight from the RO filter. I used pool filter sand in the tank and at a few inches got 4 TDS. All good so far as I know. I added some well seasoned drift wood from an older tank together with 1small bit of lava rock and a couple of very small Java ferns and some moss all seasoned stuff. I got to the desired 50 litre level and started to add some re-mineralizer. Barely made a difference at 10ml and TDS 14. So added 50ml re-mineralizer. TDS reading skyrocked to over 240 TDS way above the 100 TDS I was looking for.

I then drained the tank to almost sand level and started again with fresh RO. Still in that process now but have removed plants and wood and testing seperatly to see if that caused a problem in their own RO water to see if something is going on there.

Any thoughts whats going on here.


First of all: When using products to remineralize, better work with GH/KH and not TDS for measuring. TDS includes everything: GH, KH, Ammonia, Nitrates, tannins, salinity, ions, even water conditioner. Whatever is in solution in the water. As it makes no distinction it only helps you when you know the composition of the singular parameters.

As an example when you know 2ml of a product equal 1° of GH and KH, add enough until you reach the desired GH and KH, THEN measure the TDS, so you know the base level you have to reach.

Also when you measure Nitrates or the like you can subtract them from the TDS, like let's say you got 200mg/l TDS and 20mg/l Nitrates, you have 180mg/l TDS from unknown sources and 20mg/l Nitrates.

It takes time and a lot of testing on an individual tank to get to the point where you can just hold the TDS meter in the water and know what the reading actually means.

Oh and very important! Any stuff you mix in to remineralize has to be added before the water goes in the tank. Only that way you have control over it. Otherwise this mixing and matching is necessary each waterchange.


Thank you MacZ very well explained. I'll give it another go today.
MacZ, back into adding new RO water to the tank this morning.

Something I thought of during the night..... I added a re-used sponge filter to this new tank plus about 1/2 kg of used noodles. I keep these for seeding my new tanks. I am assuming that's where I am getting TDS readings.

I tested this tank so far with GH test kit and its starting out plain green with no colour change. According to the API test kit instructions it should change from orange initially then to green while counting the drops.

The rock test this morning in straight RO left overnight was 0 ppm TDS. However the wood read 17 ppm TDS same overnight in straight RO. Seems the wood is a tad suspect.

So going on the current GH test kit readings and TDS would this be a natural ??? Re-mineralizing of the water.

I have had problems in the past with water hardness here in Australia and always had problems getting any readings at all with the GH test. I just tested straight RO and same test. I also just noticed the expiry date on the solution bottle 08/2021..... the KH bottle was 2023. Poor packaging. Any idea how long do these solutions stay active. I'll have to find a new sourcece I think.

I appreciate you input here it is good of you.

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