Betta With Mouth Fungus

[Vee33]

I have one male betta in a 29g planted tank with a filter, but no heater. I have had my tank set up for about a month now, and everythings been fine untill about two days ago I recently added planted aquarium substrate, some banana plants, and some dwarf hair grass, along with 12 feeder guppies, so I would have more than one betta in a 29g. Last night I noticed a small white dot on my betas bottom lip, and I woke up this morning and it is completely covering his top and bottom lip. What is the best method to treat this? And could the addition of the unhealthy feeder guppies?

 

[Wildbetta]

Without a pic, I can't be for certain but I believe with your description your betta has columnaris.  This is a very fast acting illness and is very contagious.  The only real way to treat it is to get an antibiotic that treats gram negative bacteria.  A good medicine is Kanamyacin or Kanaplex.  You really need a heater with a betta.  They need constant and stable temperature of a minimum of 78F but 80-82F is much better.  Columnaris could have came from adding feeder guppies who could have been carrying it.  The main reason a betta gets this disease is from being stressed and having a lower immune system.  The temperature of the tank with no heater and then the addition of other fish to "his" territory pushed him over the edge.  Some bettas are not good with other fish.  How often are you doing water changes, how much when you do change the water, and is your tank cycled?