Jacki, when the fish is large enough and accessible enough, you could manually remove an anchor worm with tweezers. Then if it leaves a sore behind, just dab that spot with 3% hydrogen peroxide or iodine/betadine to prevent infection. Can imagine this would be a daunting prospect in this case, however..... mosquito fish are so small. The goldfish and koi references I have saved seem to advise treatment with Dimilin. So check for this listed in the ingredients of any anti-parasitic treatment product you might consider.