I've copied and pasted this information from koiphen.com, a site devoted primarily to koi, but also has forums for gold fish and turtles. This information can be found under their thread "Koi Health, A comprehensive Discussion".
SAP Treatment:
At one time it was generally agreed that Malachite Green was by far the most effective treatment. However, a treatment for topical bacterial ulcer disease has shown a very serious advantage. Tricide Neo, produced for dips and sprays has now been used for topical swabbing when simply mixed with the slime of the fish. It is important to continue to work this into and under the scales and flesh of the fish for about 5 minutes. Then simply wipe off and revive the fish. The results have been extremely good. It remains to be seen if this will become a staple in the medicine cabinet for SAP but it definitely has proven, in the short term, that it works.
The following is older information but Gentian Violet still is an important tool in the medicine cabinet.
Gentian Violet may also be used as it is antifungal. GV is available over the counter at most pharmacies. It is very inexpensive and easily applied without the mess of mixing or the dangers of Malachite Green.
Columnaris: Mouth Fungus; Fin Rot, Cottonmouth Disease:
This is not a fungus or a mold. This is a bacterial infection-gram negative. The problem is that it can somewhat resemble SAP. Symptoms include grayish white spot on the head, gills or body sometimes surrounded by a reddish tinge. It can be topically treated with Iodine and then given antibiotic injections per the charts listed in that section. I would, however, strongly suggest the use of Tricide Neo as the topical of choice when used as a paste mixed with either slime coat, or even better, Silvadene.