I would start new plants too. I know waiting for new seedlings isn't fun. But it's just about imposable to 100% kill bacteria, fungi, and/or pathogens on the plants roots without killing the plant first. By using the UV light, and/or regular doses of H2O2 you may actually be able to control it fairly well. However it will never really be out of the system when reintroducing the same infected plants to it.
P.S.
I'm interested in the UV light you got. I haven't looked for them at pet stores. Do you have a brand name, product number etc.? Does it have it's own water
pump, or is that separate? What's the flow rate? how much was it? Are they used for fish tanks or something?