Until discovering this method of treating tail rot, I had previously run into one problem or another with any other method of treatment I tried…either the medication did little or nothing to cure the tail rot, or it worked fine on the tail rot but damaged the health of the guppy itself.

Having gone through a wide variety of formulas to cure the bain of the guppy breeder I gradually reached the conclusion that anything that was strong enough to cure the tail rotbefore the show value of the guppy was lost, was too strong for the guppy to live in safely.

The answer had to lie in getting a super strong dose on the tail area without giving the guppy the same dosage. Since this theory obviously does away with adding medication to the tank water, the next locical step was to net the guppy out of the water for treatment.

To make a long story short, several methods and several types of medication were tried. The easiest and most effective method turned out to be as follows:

1. Net the fish and hold it in the net until the first wild thrashing aroung stops.
2. Hold the fish firmly but gently within the net so his head is uphill from his tail… this is vital as thestrong medication must not run down in to the gill areas. Should this happen, immediately return the fish to the tank and cross your fingers.
3. Paint the exposed side of the guppy tail, beginning at the peduncle and concentrating on their final fringe where the tail rot is most active.
4. Paint the other side of the tail right through the net from the outside.
5. Wait about 20 seconds after painting then return the fish to his home tank (providing of course, that it is clean and well cared for).

All signs of tail rot should be gone by the next day and within a week new growth should be visible. Occasionally a fish needs more than one treatment (maybe a spot was missed during the first treatment). Because the functional part of the guppy does not come in contact with the medication it has proven perfectly safe to repeat the treatment as often as necessary to clear up even the most persistent tail rot.

The medication that has proven most effective on my guppies is Tetra care Fungistop (active ingredients: colloidal silver, natrium chloratum, Ma-gn.sulphuric) used full strength from the bottle painting directly on the tail it seems to stop the tail rot right now and regrowth begins quickly… no need to trim away diseased areas. If the tail rot has not been allowed to progress very far, the fish is back in show finnage in no time at all. And since it is not necessary to add medication to the tank water, the fish is not weakened but remains hardy and vigorous. It has certainly proven to be the quickest, safest method I have come across.

Source;  Article Fish Talk