What kind of fish do you have? What is their optimum GH?
Always include the units when you are reporting test results.
You need to approach it slowly. Fish cannot tolerate too fast a drop in TDS.
I would set up a series of water changes that will drop the TDS no more than 10% with each water change, and do no more than 2 of these per week.
Lets say you use RO, and do not add anything to it.
A 10% water change would drop the TDS 10%. Simplest way to handle it.
If the aquarium needs larger water changes for any other reason, then test the water and set up the new water to match the current parameters. (TDS, GH, KH)
Do not add anything to the tank that would add a lot to the TDS. If you are doing the EI fertilizer method, I would cut the dosing to perhaps 1/4 or less, or else switch over to the PPS-pro system.
The other problem is that when the pH is under 6, and the KH so low, the nitrifying bacteria are not very active. I would try to raise the KH just a little bit, hoping to get the nitrifying bacteria population back up. Add potassium bicarbonate or sodium bicarbonate to the water to raise the KH perhaps 1 German degree of hardness with each water change. See what 1/4 tsp of either will do for you, perhaps a rounded 1/4 tsp. If you can get the KH up to about 5 German degrees of hardness that ought to be enough. Monitor the ammonia. While the pH is really low, ammonia is in the charged form, ammonium, and the fish can keep most of this out of their system. As the pH rises more of the ammonia is actually ammonia, and is toxic to the fish.
I see in your signature a link to a 20 long. In that thread you discuss CPDs. Did you end up with them?
Celestichthys margaritatus ? Celestial Pearl ?Danio? (Danio margaritatus) ? Seriously Fish