Just an observation here, but you said that you have 20 snails in a 33 gallon tank. Snails need a constant food source. They also produce waste. Eventually your system becomes mature enough that you may not be producing enough algae as a food source and they will slowly starve. You can watch many videos online about the issues surrounding many "clean up crew" packs being huge overkill and most just dying off slowly which will cause spikes in nutrients.
To put your 20 snails in perspective, I have 8 snails (4 varieties) in a 25 gallon tank along with 4 small hermits and a single emerald crab. The tank is really clean for the most part and the snails are all very happy.
Keep a close eye on all 20 snails.
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