Weird white spot on male guppy

Question
I have a medium sized glass bowl (probably around 1-2 gallons) with no filter or pump. I have one male guppy and a panda catfish, glass stones on the bottom instead of gravel and a small bunch of what I think is Hydrilla verticillata.

I try to do almost a complete change of water once a week because the water gets really cloudy.  I'm not sure what the pH, ammonia and nitrite levels are.  The bowl gets a lot of sun as it sits in the living room on a coffee table.  I assume the temperature in the tank is warm as it's usually around 80F here (in Hawaii.)  (It's around 88.8F right now if I can trust my wobble clock...)

I've had the bowl for about two weeks.  I originally had a different male guppy which died after a couple of days after I got it around two weeks ago probably from stress or getting stuck from the glass as I didn't know you weren't suppose to do a complete water change.  

I changed the water and put in a new guppy this past Saturday.  I changed 3/4 of the water last night and when I went to feed the fish tonight the guppy had a large white stripe (sort of like a long thin oval) on the back, near the tail fin.  Not sure if it was there this morning as I was in a rush.  I don't think it was though as I think I would have noticed it since it's a dark gray guppy.  

I think it might be ich, but I'm not entirely sure since I've never seen it before and the weather here is pretty warm.  If it is ich will I need to separate the guppy from my catfish and will I need to get new plants?  I'm not really sure what to do and any treatment wouldn't be able to happen until this weekend as I'm not able to get to a pet store any sooner than that.   Any help would be appreciated =)

Answer
Hi Meg;

The bowl is just too small for two fish and it's getting too warm for a guppy to be comfortable. Guppies are cold water fish that need to be under about 72f all the time. He is stressing out and may be developing some sort of infection. The panda cory cat is tropical so he doesn't mind it as much but it's still pretty high. The higher temperatures are going to reduce the oxygen saturation in the water too so you will probably see the catfish darting to the top for a breath of air more often than he normally would.

In an unfiltered tank or bowl you can have one inch of fish for every two gallons of water. And that's only if the fish can tolerate low-oxygen unfiltered conditions. Bettas and cory cats are the ideal fish to have in there because they both have special breathing organs to store oxygen when their gills can't take in enough. That's why the cory has survived. It's still pretty small though and far too warm for two fish to survive.

If you could move the bowl to a cooler spot and change 25% of the water every day it might keep the guppy alive for awhile longer. Your tank/bowl should never be in full sun. Keep it shaded. He may need medication for the spot too. I would suspect a bacterial infection. Look for medicines at the local fish store that match his symptoms.

I hope he does okay...

At Your Service;
Chris Robbins