Dwarf Gouramis don't really need a good filtration going on. They come from slow moving waters and the little gunk doesn't really bother them. They aren't very good community fishes and would be good in a tank with their same species. What they do need to thrive though is soft slightly acidic water. A pH of about 6.5. The white fungus you see in the water is mould on organic matter. In a healthy set up this shouldn't occur. You could be overfeeding and you might not have enough water circulation in the tank. To add to that you're cleaning your filter media a bit often. This would be fine if you use old aquarium water instead of tap water. A combination of this practice, overfeeding + washing out the filter with tap water = ammonia. Ammonia is toxic to fishes and is what's proabably stressing and killing your fish.

Try to feed live/frozen foods. Frozen blood worms do just fine. These pollute less and fish tend to finish this up more cleanly. Live, fine floating plants like riccia fluitans/lemna minor will give them a bit of a shelter and will encourage them to build bubble nests and breed as well as help soak up nitrates and keeping the water healthy.