hmmm I dont think in a few months the water is going to be your issue seeing your plants in the windowsill.
big floofy catts big floofy intergenerics big floofy plants need big floofy spaces.
go smaller next time you will be much happier with the outcome
use the right fertilizer for it and that's the best you can hope for.
Rainwater will require a different treatment for using fertilizer bec the ph is typically lower and into the acidic range.
Your tapwater should be nuetral. And why it doesnt do anything for you or your plants