Although I've had fish tanks for 10 years, I am NOT an expert.

I use tap water. Put it in a bucket, add Stress Coat, which ALSO takes care of the chlorine and such, then put that in the tank.

I put in a bit of aquarium salt whenever I change the water.

I use CYCLE to get the right invisible things into the tank.

Twice a year, our city does something to the water and I have to do a bit more to adjust the PH.

I do think you should have one filter which is appropriate for the size tank. I normally get one size larger if it isn't a lot of money extra. Think about getting that done! It is a factor in PH levels, and you will need to correct the filter sometime, may as well do it now.

Here's a thought. Can you put some of the 10 gal water in a bucket to hold it, add water to that one to get it back up. Do that 2 or three days in a row (yeah I know, lots of buckets of water around to spill.) Then add THAT to the larger tank? You will still have the 10 gal tank ready if he has to be moved back again.

Silly dragon Falcor, making you go to all this trouble for him; he just KNOWS he is special, he he.