Wound on Kuhlis belly

[Kendallkemmer]

Please help, my Kuhli Loach Mr. Wiggles suddenly has a wound/ulcer on his belly. I have him in a quartentine net and while he is breathing is just floating on his back currently. I dosed the tank with Melafix and Pimafix. What should I do?
I tested the tank as soon as I realized he was injured, there's no ammonia or nitrites, but the nitrates are at about 20 (on that note, I was recommended to add cleaning bacteria at Petco, is this the right thing to do?)Temp is set to 75 but is closer to 78 due to the heat. PH is at 7.1. The tank 36 gallons, filled with maybe 25ish gallons (I'm in an upstairs apartment) and is only about 3 months old. I've got two sponge filters going, a 30 gallon one and a 50 gallon one. The tank has 6 mollies (they're 2-3 in each), two mystery snails, their countless babies (I know it's not ideal, I'm working on it) and of course the 4 kuhlis. Also possibly a few bladder snails, every time I think they're gone I find another. All of the fish have been in the tank at least a month. This one in particular probably 1 and 1/2 months. I did a water change about a week ago and normally do one every two weeks (more often if necessary) I usually do about 30%. There are several plants, a large piece of wood that they like to hide in, and a couple of rock caves (plastic). The most recent thing I've changed was adding a small cassoral dish filled with sand, which I buried in the substrate (which is a smooth gravel) but this was about a month ago (shortly after I got the other 3 kuhlis. The kuhlis mostly eat the algea tablets and carnivore tablets by hikari. I've seen them eat baby snails before too. The tank as a whole also gets omega flakes, frozen cubes, and Snello. Actually they also really like the Snello, which is a new addition as of two weeks ago.
I added a couple of floating plants in with Mr.wiggles and turned off the tank light. I put in a little food too. What else should I do, he's my favorite (possibly because I havn't even named the rest and I've had him the longest). The only thing I can think is maybe he hurt himself on the wood? I hadn't moved anything recently and he was fine yesterday. I've never seen the Molly's bother them and it's a big spot to be from one of them.
I've included pictures.

 

[Autumnsky]

Thank you for providing a bunch of details.

How did you cycle your tank?

Are you familiar with the nitrification cycle?

What are the ammonia, Nitrite and NitrAte levels, pH and any other tank parameters?

I would start with getting a couple water changes done asap!

No mater if the water is "okay". The wound cause is not exactly known to me.

Is there anything in th tank which could cause an injury? Kuhlis are notorious for squeezing into any place even places, seemingly too small for them.

Holes in decorations even ones underneath the decoration.

Check any and all decorations for anything possibly sharp /pokie.

But if the tank has only been set up for less than 2 months, the water quality could be an issue, if the tank is not cycled, or there were a bit too many fish added for the amount of BB in the tank.

The signature link below about getting started is really helpful and explains many of the issues about running an aquarium.

The fish-In cycling articles linked in that one below are useful as well.
It could have come from a puncture, scrape - from a physical

 

[Kendallkemmer]

Hey, thank you for the response. My Kuhli died pretty quickly after a posted this. Most of your questions are already answered in the post. I cycled it by running the filter for a couple weeks and then adding a few snails when things started to steady out. I think it must be the wood, it's the only thing that could be sharp and it has a lot of crevices that they like to swim through.

 

[Autumnsky]

I am sorry about the Kuhli.

Building a beneficial bacteria / BB colony takes usually 6-8 weeks. This is needed for the tank / filter to process the tank water waste in a safe manner to keep the fish well.

Still suggesting the article in my signature (from this website) to help for the future tank inhabitants and the health of the tank.