if your city water is 30 even a half assed ro unit should be able to pull your tds down to 0. it sounds like the store might need to change their inserts in the ro unit. you should consider getting your own unit they are pretty cheap these days. question though, are you storing your water in a food safe container? if you stored (or the shop) in a non food safe bin or a dirty bin then your water might have been zero and the container shot the tds back up. you must use food safe containers to store your ro water. non food safe bins can also leak phosphates. also if your testing your tank water for phosphates and you have algae you will always test zero because the algae is using the phosphates to grow.
if you did disturb the rock and bed you probably shifted all kinds off stuff up. im sure everything will settle down soon. if your algae problem continues you should consider a phosban reactor, it will pull phosphates from your water helping eliminate the algae problems.
btw welcome
