My pond always needs at least an inch of water every week during hot weather (vol is about 800gal) - water is lost not only from the water surface but from things like cascades, and also through plants, which if kept permanently moist (eg on a shelf in the pond)grow furiously and lose water (well mine do!)- but this loss is of cause minor compared to evaporation.

Just do the obvious and check all connections for tightness.
Once Autumn comes you can probably get a better look at the pond, when the plants have died down.
Stew

Hello, my huge pond (43ft long, 13ft wide) is practically empty (from 2feet to 4inches deep now) - thanks to no rain for months. I haven't topped it up because I think tap water would kill the wildlife.
Praying for rain!

Stew, thanks, that's reassuring. There's nowhere for the plumbing to lose water - by design, leaks from faulty connections would stay "in the system", so the only possibilities would be a hole in the hose from pond to filter (this would be easily spotted); an overflow somewhere in the pond return (again, easily spotted); or a hole in the pond liner AND the one under that, from the previous pond incarnation).

The pond liner is thick EDPM rubber so it's unlikely to have a hole... however, my cat has (hilariously!) fallen in the pond once and could have made a hole with her claws in her scrabble to get out. And she might have fallen in other times when I wasn't there to see it (I've seen several near-misses when she's chased flying insects and forgotten where she was...!)

Kate, ah, well I use a dechlorinating water treatment that's meant to turn tap water into pond-safe water... I have lots of "wildlife", plants, fish, and a bio-filter and they don't seem to be affected. I just stick a water butt next to the pond, put some treatment in and fill it up from a hose... then turn on the tap on the water butt. But then that's easy for me because my pond volume is much smaller than yours... I only have to do that 2 or three times each top up. I reckon you'd be at it for days doing the same with a pond like yours, not to mention the cost of the water treatment.

I have lost about 1/2" over the last week and it hasnt been blazing hot plus we have had the odd shower so your losses are not unreasonable.

Thanks for the thread, I have been wondering about the issues regarding topping up. My pond is a good sized, wildlife pond (no mechanics) and of course there is evaporation when it's hot. I can run a hose from the rain butt to top it up, but I can only do that once between rainstorms. I tried avoiding using tap water otherwise, but it does go down quite a bit. There is a pebble beach which homes quite a lot of creatures, including the tadpoles which are still quite numerous, and I have been relunctant to let that area dry out, so this year I have topped up with tap water as well, a little at a time. So far that seems to have been ok.

I know more exotic fish are very sensitive to water changes, but does anyone know what the risks are for wildlife? I am very relunctant to use chemicals anymore. I'm not adverse to a greenish pond, as long as the critters are happy...but if the aren't...

I have found that a little topping up with tap water doesn't do any harm. I guess it all depends on the area you live in but Manchester adds chlorine to its tap water.
When the fox caused my leak, digging its claws in after froggies I had to run a hose in for about an hour, but I did add a conditioner during this major top up.
I don't run any mechanical pumps or filters and my wildlife don't appear to suffer !
Dave