60L, that's 15 U.S. gallons, I think. Way too small for two goldfish IMO. When you say that the tank is mature, what exactly do you mean by that? Did you do a fishless cycle? If it is a new tank then I would assume that it's not "mature" unless you either fishless cycled with ammonia or something like that, or used Biospira, or used gravel or media from an established tank. If it is not cycled and you have two goldfish in it then you are probably having some sort of problem with your water quality such as ammonia or nitrite. I'm not positive that that is what is causing the foaming, but it is a place to look. Let us know what you find out and hopefully we can help you out.