Question for the experts...?

I'm pretty sure my fish have columnaris. How they got it, I just don't know. Anyway, I have some maracyn-two. Can I use that or should I get something else to treat this? Also, what can I expect for results and how do I prevent this from happening agian. I do regular water changes (50% AT LEAST once a week) and add aquarium salt to the tank.
10 gallon tank
1 - better
2 - neons
1- very small pepper cory (he looks the worst!)
Answers:
Columnaris is as you are aware a bacterial infection and can easily be spread from fish to fish and can seem to wait in hiding ina tank and attack a weakened fish.
Maracyn 2 is effective against external columnaris infections, but if possible I would suggest you treat with Furanase, Furan-2 or even Maracyn TC as they are better absorbed by the fish and would help in case this infection is also internal.
As for preventing it, a quarantine tank is your first and best line of defense against this or any other disease making it into your tank. Once the fish are in your display tank, good water quality and good diet are the best ways to help prevent disease.
If I can help further, feel free to email me.
MM

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of course cory looks the worst - they cannot have salt water!
anyway, columnaris bacteria is always in the fishtank, but only stressed fishes have an immune system so weak that they get sick.
betta and neons also don't like salt very much.
while your fish tank isn't yet cramped and you do good water changes (thanks for it, many of folks writing here don't manage to do that), the stocking of your tank is not very optimal:
neons are schooling fishes and should be kept in a school of at least 10.
corys are group fishes and should be kept in groups of at least 6. if they are alone they are frightened and thus stressed so they get ill easily.
if you want to prevent the fishes getting ill ever again while keeping the same fishes you keep now you'll have to do quite a lot of things. but as soon as you do it you'll see that you suddenly don't have to do as much work with your tank as you did before and fish keeping starts to be much more fun.
first of all, you'll need a larger fish tank (let's say 30 gallons) with a large external filter. as a large filter would be too strong you'll have to reduce the output of it on the outlet (not at the inlet as it might damage the pump).
then you should put a strong uv sterilizer after the filter. most pet shops suggest a weak 9 watt one for a 30 gallon tank but it won't suffice. go for at least 36 watt.
then you get the finest aquarium sand and use it as the substrate, first putting a little water into the tank and then putting the sand in, then fill the new tank up, plant it well, cycle it for 4 weeks, also waiting for the plants to grow. check the water data every week with a real test kit. you should have a soft, slightly acidic (ph 6.7) water. if you don't you can add some oak extract untill the water is perfect - corys and neons do like soft and slightly acidic water and columnaris doesn't, it prefers hard alkaline water.
so, now you can get the fishes. one betta is okay, 12 neons are fun, also 8 or 10 pepper corys and a couple of apple snails.
now you have got a good working fish tank with good conditions. all you have got to do now is to feed the fish (always just a little bit), change one third of the water every 10 days, fertilise the plants after every water change, check your water regularly so it stays soft and slightly acidic and change the uv lamp once a year.
that's it, if your fishes are healthy in the first place they should never get sick again. no big water changes anymore, no hassle with salt, no need for medications, no algae, but instead happy and healthy fishes