foamy water If your aquarium has an aeration or splashing water, you're probably used to some foam or froth from normal water agitation. But if that foam keeps accumulating and doesn't dissipate, spreading like a white carpet, it may be a sign of an underlying problem with your aquarium water conditions.

surface scum - aquariumalgae_blogspot_com - dusko bojic Foamy water / Surface scum is bubles accumulation on the water surface that won't break easily. Stubborn bubbles are commonly due to an excess of dissolved organics and/or proteins collecting in a slick along the water's surface. This slick covers the bubbles and make it difficult to break !.

Bubble source may come from aeration, venturi or splash water.

Many factors can cause this problem, usually unfulfilment of good water parameters. You may check some points :

Use water testing kits to check for ammonia, nitrite, and nitrate. Some amounts of nitrate are acceptable, but if any ammonia or nitrite are detected, your tank may not be cycled properly and you'll need to add more aquarium filtration or additives to bring the aquarium water conditions back into balance.
High nitrate can also mean that you keep more organisms in your tank than your filter can handle.
Filter size. If your filter size is too small for your aquarium, some of residual organics from fish waste and plant decay can float on the water's surface in form of slick. Surface agitation to stir up the surface water, proper water circulation to bring the waste into filters and getting a larger filter are needed actions to overcome this problem.
Dechlorinator that used during water changes to make your tap water safe can also cause foam buildup.

Foamy water can also be caused by Changing water & cleaning the biological filter at the same time :

Water change reduce pH level. If pH low enough (pH<7), Ammonia (NH3) can be ionized by the aquarium water becoming Ammonium (NH4) which is harmless to fish. But too much ammonium can cause surface scum.
While cleaning the biological filter, especially if you clean all the filter media (not partly) can reduce the bacteria significantly. Lack of bacteria to do the aquarium cycle process, makes organic particles / proteins floats on the surface.

So, it's better to seperate above activities in a weekly interval, also clean your biological filter media partially by using filter bags to keep enough bacteria population.

I hope this article can overcome your problem .