I can only suggest to use isopropanol (not sure whats the english word but i think you guys will know). The good industrial stuff with high purity. You literally only need to fill you bong with like 100 ml, shake it around a little, rinse with it water afterwards and it will clean as fuck. I just cleaned my bong for the first time after many years and it took like 5 minutes to have it look new again.
I can only suggest to use isopropanol (not sure whats the english word but i think you guys will know). The good industrial stuff with high purity. You literally only need to fill you bong with like 100 ml, shake it around a little, rinse with it water afterwards and it will clean as fuck. I just cleaned my bong for the first time after many years and it took like 5 minutes to have it look new again.