If you are sure its white spots and not bubbles then its probably ich. You can't add any salt to the water as you have a plec in there and it will harm him. You will need to get meds for ich and start treating straight away as its contagious. You could also give your betta a salt bath in a tub or anything that will hold it and some tank water and be able to float it in the tank so it keeps temperature. There is a link Salt in Aquariums to using salt for treating diseases and on how to do a salt bath
Hope your betta makes it. I've had ich recently in 3 of my tanks, one containing bettas. I also had scaleless fish in so couldn't use salt so did salt baths on my girls that had it. It can be a long process, I'm into my 4th week of treating the 210 litre tank but today I don't see any spots for the first time
/www.fishforums.net/index.php?showtopic=81865
