Consider:
If you use a powdered chemical (or flakes), dissolve the stuff in a small amount of water first. Doug's idea about organic material is spot on.
If you decide to do this next suggestion CLEAN ( rinse a lot) out any previous chemicals. Then try Clorox. Assuming what you see and want to get rid of is organic, bleach will break it down fast, just empty the vessel out every hour and then add more bleach. Should have most of it gone in a few hours. The brown stuff you see in potting soil residue on glassware is largely organic. Glassware often develops a patina of blue green algae on the inside when used as a pot. That breaks down with bleach, too. Do not reuse the vessel for small fish. Fish and even small amounts bleach in their water are very incompatible.