As Dave posted, Do you know what to look for and do you have what will be needed to treat right away. In most cases if you find someone wanting the need for a rescue your probably also going to find deteriorating pond conditions. Also be prepared, Your going to need to have the availability to house large fish as the pond may have been set up for several years and may be the last one or two survivors, That's the good side, bad side is you come up on a green pond and you go to seine and find it has several hundred ranging from a few 1-2" to five or six over the 2' mark, All sick and malnutrition but how were you to know until you got them out? Now your responsible for their care. I am very familiar with each of these examples. I lost all my fish in one pond this past spring, I was told of a couple of koi that needed rescuing, one pond led o two ponds led to three ponds. Space ran out and what happens? yes a forth person needed help and this time a paying client. 300,000 gallons of water, 8 ponds to work with, and yes I am out of room. I've treated more fish this summer than I have treated in the last 20 years I have been in the hobby. I've lost only two out of over 100 fish rescued, yet, only 10 have been re-located so far with 8 more spoken for. So what do I do with the other 82? I will happily keep 30 of these as I actually got some very nice fish in all this too. But some are really big and I want to give them the best care I can. It's not easy finding homes for 50 10"-18" koi, food bill isn't cheap. medication add up fast, extra water changes, yes, bigger water bill, heated 300 gallon tank, Holey moley it take HOW MUCH electricity to heat a 300 gallon tank to treat for how long? I commend you for what your wanting to achieve, BUT, be careful, look smartly, and remember, no matter how hard you try, you can't save them all.