25gallons a week is not all that much. I water like every 3-4 days, so about twice a week. Every time I water I use about 12-14 gallons. So in a week I use about 25gallons of water.
Each plant gets like 3/4 gallon to 1 gallon of nutes every watering. THey are in 2-3gallon pots, and I end up with about 25% runoff. By making sure I have this runoff, it keeps me from building up salts in my soil.
And as far as the fish tank goes, you never want to change all the water in it. You want to do like 25% changes every week or two and top off as needed. You will kill your fish if you change all the water at once. The fish need a slime coat, bacteria, etc in the water, and with fresh water, there is none of this stuff.
When you first get your tank, only get a couple of cheap fish you are willing to have die. Put these fish in the tank for like 2 weeks. These fish will get the ecosystem going in the tank, then you can start adding other fish. I have had fish tanks for the past 10 yrs, and kept fish alive for up to 7yrs.