There's a few easy and painless things you can do to reduce the amount of water you use.
1 - don't run the tap while cleaning your teeth (as already said)
2 - put something like a 2 litre pop bottle filled with anything into your toilet cistern, this reduces it's capacity and it will use 2 litres less each flush
3 - (not a fan of this myslef) don't flush every time you go to the loo "if it's yellow, let it mellow, but if it's brown, flush it down"
4 - re-use whatever you can, so use your old tank water to water your plants and garden, it contains fish poo, nitrates and phosfates etc, they will love it. if you have a dehumidifyer, use the water it collects in the garden or to wash the car.
5 - even if you shower you can save water, when soaping up, turn the water off, then back on to rinse you off.
6 - don't fill the washing up bowl to the top unless you need to due to a large load of pots to wash, use half a bowl and stack the dirty dishes on one side.
7 - if your washing machine / dishwaser allows it, use the half load button, conversley only use the washer to do full loads
If it does start getting serious, reduce the feeding of your fish slightly and reduce your water changes, either change less so do 20% instead of 25% or change every 10 or 14 days instead of 7 days.
Arfie