never had the bubbles but I did have my stopper come loose and leak fuel all over the place after just a few flights, also make sure the clunk line in the tank is on the bottom and not tangled up inside the tank 9 out of 10 times bubbles are from a air leak.
my NEXTAR has tons of flights on it and does not have any foam around the tank. the tanks supplied with this plane are not the best so its possible there is a hole . try this, take some new fuel line put it on the fill tube and plug the others put the tank in some water and blow into the line if it is leaking you will find out fast , also double check to see if the lines are hooked up properly inside the tank the one I have is solid white so you can't tell unless you unscrew the stopper even if it seems like its O.K. best to make sure, mistakes do happen during assembly and hoses get switched it has happened to a few on RCU .I did this to myself switched the lines on my tank after fixing the stopper the engine would run because there was plenty of fuel but would quit after a minute or so realized I had hooked up the fill line to the engine after mixing up the different colored hoses . that was a long time ago LOL