Pond pH and backswimmers

Nothing is as straightforward as you would like it to be, is it. I tested the pond water last week and it was decidedly on the acid side of things at 6.2 (most fish and other aquatice life, although there are exceptions, prefer a neutral pH of around 7). According to the fish guy at Wollongbar, rainwater running off an iron roof is quite acid (and in fact when I tested some water in the rain gauge yesterday it was 6.1). But, he said, ground water (like the bore water that I thought would be inferior to the rainwater, stupid me) is quite good. So I tested the bore water yesterday from one of the outside taps and it was spot on 7! So we syphoned out most of the water from the pond this morning and filled up with bore water. And then I tested it and it's 6.4. What the...? Can't work that one out...anyway I'll test it again tomorrow (maybe it just needs to 'settle') and if it's still low we'll drop in some concrete blocks which should do the trick of increasing the alkalinity.

When the water level had dropped we noticed quite a few tadpoles, presumably an outcome from the orgy of bleating tree frogs that took place earlier in the week, as well as some backswimmers or water boatmen, not quite sure which. Now I'm buggered if I know how they got in to the pond. I guess they could live in our water tank and came in via the hose when we filled the pond up a couple of weeks ago, but I would have thought we would have noticed them before now (the pond water is very clear). Who knows, another of nature's (NOT Mother Nature's) mysteries...