That's your black carbon then that removes chemicals, but if it's been in four weeks it want's replacing with sone new carbon that will get rid of the coloured water.
Not familar with your type of filter leave it alone then if you only have one sponge, the water will clear over time with water changes, is it a very small filter.
Ammonia should be 0 so it sounds like your tank is cycling, well to be honest the neon tetra's were not a great choice as they need mature tanks of 6 months, as they are very sensitive to water quality, i would increase aeration, good luck.