# Targetting a lower TDS

#### saqib

Hi Guys,
My tap water/source water is fairly hard with a TDS of ~280. Has a high level of calcium in it, you can literally see crazy water marks as the water evaporates from the tank & even from the surfaces where the water may have dripped whilst doing maintenance.

From the limited research that ive done an ideal TDS level should be around 120 for the longitivity of the plants and the fish., im building a heavily planted tank and to eventually have discus in it, so planning to bring it down slowly by mixing RO water with every water change.

I've done some calculations, is the following correct?

 Tank water TDS 262 Litres in tank (Total) 280 RO water TDS 8 Water Change (Litres) - Max capacity of my RO storage tank 80 Water Change 29% TDS Coming from RO 2.3 29% of RO TDS coming from remaining tank water 187.1 71% of existing tank water Effective Tank TDS 189.4 TDS of Tank after water change

#### MacZ

You are overthinking and overcomplicating things. Trying to get to a certain TDS level is as useless as aiming for a certain pH. Also you leave out that TDS includes ALL solved solids, so KH (carbonates), NO3 (Nitrates), and all the ions of iron, magnesium, potassium... all that counts into TDS. Measure GH, KH and at least NO3, so you know whether you will have to add fertilizers or something for the plants.

But for pracitcal advise: Always go by the original TDS of your tap. Assuming your RO unit is working correctly the RO should have neglegibly low TDS of below 10, so just for the calculation go with zero.

So the following calculation would apply:
280mg/l : 2 = 140mg/l, so to get to 120mg/l you only have to cut the tap with 55-60% RO. To switch the tank to that level do several waterchanges with that ratio premixed. It's easier going with ratio instead of exact numbers, because that will only lead to mixing and matching.
Then finally, when you replaced the whole of the tank volume your TDS in the tank should be where you want them.
(And before you ask: ppm and mg/l are the same, I just think mg/l makes it clearer TDS is not one thing alone.)

For the fish themselves (except you are about to buy Stendker Discus) I would rather go the blackwater route and get TDS to below 50 and only keep undemanding plants while mainly using driftwood and branches for scape.

#### saqib

Makes sense.
Mixing 50:50 is the route I'll take for now.

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