I receive a lot of enquiries about microscopes for sale.
This combo is available online at
http://shop.technopro.co.za/tasco-25x30mm-telescope-microscope-combo-49tn-p-598.html and should be sufficient for koi keeping. It will identify even Costia which is one of the smallest parasites and requires 200-300x. According to the specs, this scope can go up to 900x.

If somebody buys it, please give us your reviews. It seems like a very good price for what they offer.