Why is there foamy white substance in your betta fish tank?

AnswerFoam in a fish tank can be caused by a couple of reasons:

1 - The water has excess amounts of waste. The bigger the fish the more waste is created. Do a partial water change no larger than 30%. If you still see foam, wait another day or so for the parameters to level out, and then perform another partial water change.

Fluctuations in water parameters stress fish and lead to death. This is why you shouldn't change more than 30% of the water in one hit. Water changes of 50% should only be used in emergencies (pH crash, ammonia spikes etc)

2 - Excess amounts of water de-chlorinator or conditioner. Partial change of 30% should do the trick - ensuring you follow instructions on the de-chlorinator bottle. If problem still persists, change brands or look into Reverse Osmosis/Deionised water.