I'm after some advice regarding carrying water changes while I have two of my
Hzebra's in One cave

Three long years I don't want anything to go wrong !
I was thinking of just doing a 40% change or less as I usually
do 50% every 5 days but for the last couple of months I have been doing two weekly changes 70%
my TDS is currently 100ppm and I have been feeding very heavily Blood worm and brine shrimp etc, and I don't want my tank to crash.
The tank is 250l with x2 eheim 2217 with powerheads
So any help from the pro's (Barbie) etc would be appreciated