hi there,
I am no expert (still very inexperienced myself) but 3-4 inch drop seems a big drop for me in 36hrs, even if very hot. we had that recently as it was because of a hole in the liner which we then repaired
so i would check for that
i add stuff to remove chlorine when i add tap water, but i have heard that if you let it spray off something that dissipates a lot of the chlorine.
am sure someone more knowledgeable can help you more-but i would definitely check for a leak.

The stuff you take to be corals is called "alfa grog" and is one of the best filter media avaliable.

The first thing you need to do is check all your conections from pump to filter, the filter it self, then the return to the pond, a 3-4" drop is a huge ammount to lose in a short time, and most loses such as that happen from leaking joints or connections, or a blocked filter that over flows.

If you have a waterfall look closely at that, plants can grow quickly and cause water loss over the sides, but large water loss quickly is always a leak somewhere, you jsut have to find it, in dry conditions its easier, you look for a damp patch, but after all the rain we had you will have to look closer.

thanks for the tips hotcookie and ajc

having followed your advice, I've checked the stream and all connections and can find no obvious cause for the leak - so I'm pretty sure there's hole in the pond liner. My pond has a 3ft high brick retaining wall at the far side (our garden slopes down the hill) and there are no signs of water there, so I can only assume the leak is escaping underneath the pond.
I need to find and fix this leak, as topping up from the tap is not a viable long term solution, but where do I start?

Firtsly, I'll need to find a suitable safe place for the 30-40 fish (some of which are upto 20" long!!) - any ideas? I'll need a pretty big paddling pool!!
Secondly, do I forget the leak in this liner and lay a new one over the top?

oh the joys...

thanks again.

If it always goes down the same amount ie to 4 inches below, the leak is along this line.
Mark that level with tape or something in one spot.
Leave the pond another week or so for evaporation or until it has gone down another inch.
Then try to examine the exposed liner for damage.
If you find just one small tear you could try to repair it with pond repair tape.

If you need to put in a new liner, I assume it would be better for the fish if the weather is cooler rather than warmer....but I don't have "proper" fish so I'm not an expert.

ps do you have any photos of the pond you can upload onto the gallery?

thanks for advice.
I can't say for certain that it always drops by the same amount as I've been nervous about letting it drop too far - but I have ceased topping it up as of Wednesday morning (31st) - will sweat it out and see how far it drops - will post updates on this thread.

will try and get some pics this weekend and upload them.

dampflippers advice is a good one, it can save a lot of time and work, try it out first, if the water level goes down double the last line then a new liner would be advisable, sometimes you can get lucky and find the leak, but often its difficult, but worth a try, good luck.

as for moving fish in warmer weather, its better to do so if you can work fast and have a decent sized holding place while you work, you may lose one or two through stress, but you would probebly lose more long term by adding so much fresh tap water all the time, laying a new liner on top of an old is a matter of choice, some say yes some say no, personaly i have done so in my small pond, but wouldnt in my koi pond.