Please help!
I got a new betta yesterday, and he seems ill already
. My tank is 48 litres and has been cycled. There are 10 pygmy corys in with him that were added a week ago, and they currently seem fine, very active. Ammonia and nitrite are 0, pH is 7.6, and temperature is 26 degrees. I haven't tested nitrate today, but it's normally about 6 or 7 as I have live plants. I also added some wood with plants on yesterday, I'm afraid I don't know what the plants are. I did a 20% water change yesterday morning before I bought him.
He was quite active when put in the tank yesterday, swimming about, and he looked healthy. He ate a few betta flakes last night. This morning I noticed some white stuff around his mouth, and over the course of the day he has become increasingly lethargic. He spends a lot of time still, either hanging around in a corner of the tank, or sitting on the bottom. He doesn't even care if the corys swarm past him, and I haven't seen him flare. I have dosed with some Melafix, as this is all I have to hand. Should I do a water change? I think I'm not supposed to if I keep using the Melafix.
Does anyone know what is wrong with his mouth and what I should do? I don't have a hospital tank, I'm afraid, so any meds I add will affect the corys. Photos below, sorry they're not better quality, but the first one shows his mouth quite well.



His name's Hector. Many thanks.
I got a new betta yesterday, and he seems ill already
He was quite active when put in the tank yesterday, swimming about, and he looked healthy. He ate a few betta flakes last night. This morning I noticed some white stuff around his mouth, and over the course of the day he has become increasingly lethargic. He spends a lot of time still, either hanging around in a corner of the tank, or sitting on the bottom. He doesn't even care if the corys swarm past him, and I haven't seen him flare. I have dosed with some Melafix, as this is all I have to hand. Should I do a water change? I think I'm not supposed to if I keep using the Melafix.
Does anyone know what is wrong with his mouth and what I should do? I don't have a hospital tank, I'm afraid, so any meds I add will affect the corys. Photos below, sorry they're not better quality, but the first one shows his mouth quite well.



His name's Hector. Many thanks.