Bettas Mystery Illness

Question
About six weeks ago my 6 month old betta (chuck)started sending most of his time on the bottom of the tank and developed one protruding eye. Since then raised temperature to 30 (have plec so don't want to go any higher or add salt) made sure the water quality was up to scratch and been feeding more varied diet inc frozen bloodworm (fully defrosted)and skinned peas. Sad to say no improvement and now his underside has got this strange grey, green discolouration. Wondering if its maybe fungal infection from spending all his time on the bottom. Basically any advice would be brilliant. I'm at my wits end that he's suffering and i can't help.
Thanks in advance.
Kat.
P.S. 25 litre tank with filter/heater/airstone stocked with  1 plec, 1 fighter, 4 glowlight tetra  

Answer
Hello Kat,
I'm hoping that your degrees are in Celsius, not Fahrenheit.
Plecostomus do just fine in warm water, and most are actually tropical themselves. Your betta's water needs to be between 75-85 degrees F. Also, you need to get a larger tank. Depending on what kind of Plecostomus you have, you'll want to switch to either a ten gallon, or a 55 gallon. The regular Plecostomus (I'll send you a link with a pic) will grow over two feet long, and require a MUCH larger tank. If you have something like a rubber lip pleco, they only grow about 5 inches and you will still need to get at least ten gallons. Check online or with your pet store to be doubly sure you know what kind of pleco you have.
Bettas should not really be eating peas, except for small amounts when they are suffering from SBD, which I don't think he is.
He needs a larger tank, more frequent water changes, (what were your water quality tests reading at?) and 75-85 degrees F temperature.

http://www.aquahobby.com/gallery/e_Common_Plecos_Hypostomus_Liposarcus.php