It means you can have 4" of shrimp per gallon, when you can only have 1" of fish per gallon.
So one gallon could have 4 1" shrimp while it could only have 1" of fish.
A 0.25" per gallon rule would mean shrimp are 4x as dirty as fish.
Not dim, just a mix up?
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Nerite snails are pretty low waste producers for snails, I'd compare them to a 2" fish.
It depends what type of frog you are talking about, if you mean African dwarf frogs as opposed to African clawed frogs, then I'd count them as a 2-3" fish, they are pretty messy.
'Singapore shrimp' aren't that messy and would count as a 3" fish, but unfortunately feeding them is. No matter how hard you try, lots of the food will be missed and go uneaten. So taking that into consideration I would personally count them as a 5-6" fish.
Amano shrimp can be likened to small fish like neons in some tanks, because they clean up. But really if you are going to properly be using the x" per gallon rule you should count them the same as fish.
For the record, I don't use the however many inches per gallon rule
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