I've broken some backs while fixing a mess of fish. In balmy weather it's great in large bodies of water, not so much in little ponds and the cold water.

I strongly recommend that you NOT use Malathion or any other organophosphate for fish diseases anymore. The reasons are obvious including toxicity to you, and the fish and things that drink or come in contact with the water. With the advent of more cost effective strengths of FLuke Tabs, the invention of Praziquantel-contaning safe fluke treatments, even Supaverm, there's almost NO REASON to use Malathion anymore, unless you're treating a small lake. So I'm leaving this up here.....But just for small closed lakes that do NOT drain into the natural watershed.
Malathion - by Doc Johnson
Malathion is an organophosphate just like Trichlorcide and Trichlorfon, you've probably used these. But the Malathion is pretty safe (notice I did not say completely safe) and here's the skinny:
MALATHION is available as Ortho's Malathion 50 Plus Insect Spray. It has the Orthex wetting agent, and the active ingredients list reads 50% Malathion, and 50% Inert ingredients, which are Petroleum distillates.
The dose is 0.25 ppm every other day under most conditions.
Care should be taken not to use this in water cooler than 65 degrees, and no warmer than 80 degrees.
Hard water, water over 80 ppm hardness, should be dosed slightly higher, maybe even two times higher, but still only every other day.
To dose 1000 gallons, convert gallons to liters.
1000 gallons x 3.785 liters per gallon = 3,785 liters.
We want 0.25 mg per liter.
0.25 x 3,785 = 946.25 milligrams needed
There are 500mg per ml of the Malathion as described.
We will need 1.89 ml per 1000 gallons every other day.
Add it every other day for 3 treatments, then go to every THIRD day for 2 more, and that's it. Rescrape fish looking for flukes, and if none found, case closed.
"Do not use Malathion unless Fluke Tabs, Supaverm or Prazi are not available or not practical. Also, consider the information on Closantel in case you do NOT have Goldfish." ~ Doc Johnson