Why TDS increase mysteriously between water change?

[wild_discus]

Hi,
I keep wild discus.
I change my water weekly using local tap water that is 20ppm.
Every week, my TDS will increase in between water change. For eg, after my last water change, my TDS drops to 190ppm and mysteriously increases to 210ppm in a few days.
The only input I have into the tanks are hikari dried worms and frozen bloodworm.
I want to go down to 50ppm but don't want to do major water change to shock the fish.

Why would the ppm mysteriously increase? What should I do now to try to bring it down to 50ppm from 210ppm?

HELP

[Len]

Your TDS would stay more consistent if you did water changes more often. By doing only weekly changes, you are accumulating a weeks worth of fish waste and uneaten food. A large water change would be appreciated by your fish.

[the20tonknuckler]

Tds rises as the water gets dirty. Tds is normally higher in heated water then cool water taken from the same source.

[wild_discus]

How frequently should I change and how many % max each time, for wild discuss 55g
Thx

[timmy82]

So you don't shock them too much to give them a big change straight up I would do something like 25% in the morning and 25% at night then after a 25% every day or every second day. If they are already grown to adult that should be ok but if you are still trying to grow them up the water and frequency.