Hi enigmatic,

as someone who does not have water supplied and can only use tankwater, I sympathise with your problem.

You can do reduced water changes if your tank is under rather than overstocked for quite a while before probs develop. I have been on reduced water changes for six months. I am currently down to a quarter tank every two and a half weeks. Currently fish are happy, hope this continues!!

I have started using carbon. I have six discus, three medium-large plecos, one small pleco, several bristlenoses, nine cardinal tetras in a 6x2x2 tank. lots of driftwood, some plants, some sand on bottom, no substrate.

We currently save the water we use at home by
showering at work
have bought a front loading washing machine that uses minimal water
only wash clothes when they are dirty!!
don't ever let your tap just run - turn it on and off when you are doing your teeth!
you don't need to flush the toilet every time it is used, apart from obvious uses
if you can only shower at home, use a bucket to catch the water that you run until the shower is warm enough, you can use that to flush the toilet
if you shower at home, a 2 min shower is more than sufficient to soap yourself, wash your hair and wash soap off - everyone should be doing that at the moment regardless of what level of restrictions you are on, it probably won't be long before everyone is on drastically reduced water usage.
you can buy water in our area for $250 for 17000 litres

HTH