Bro, its really too over-populated.. Even if you use a 3L media canister (e.g 2213), you will need to sponges every month. 30 tetras and 3 cories will cause you super high nitrate in the long run. Moreover, you have added too many livestocks at a time. You only cycled with 3 guppies, and you added 33 new fishes. Your filter may not take the bioload, ammonia will shoot up high within 1-2 days, there may not be enough BB to break down your ammonia. Cories will also stir the sand to cause more ammonia spike. Your sponge filter will be overloaded in less than 1 wk.
Just my 2 cents.
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