Sorry kenowl,
I missed your question re filling the pond, we always topped ours up with tap water & had no problem, I would imagine a lot will depend on the number of plants you have that can take any additives out of the water & also how much you top it up by, little & often sounds good as the chemicals disappear after a couple of days as far as I know. The other thing is a water butt, the only problem here can be if you let it stand for a long time in warm weather without using it & algae builds up I think we lost a couple of mirror carp once when we used a whole water butt to top up the pond.
My biggest problem when we had 1,000s of tadpoles was remembering to turn off the water & washing tadpoles out of the pond then trying to rescue as many as I could. you may find the fish find the tadpoles good food, in the end we opted for frogs, but our carp were getting to be a reasonable size & had big appetites. A friend has smaller fish but lots of them so has a similar problem.
I wish you lots of pleasure with it, great to see someone making the effort to fit a pond into a small space; they are very relaxing & beautiful if you get some nice plants as well. Keep Irises in pots or sacks as they spread & will dominate, avoid Bull rush although I would think you would any way again they take over. Don’t be afraid to clear out a lot of plants after a year or so if any are taking over but try & let them drain back into the pond so pond life can drop back into the pond.
Dave