I used two of the smaller size of this grommet for the gravity-fill inlets into the tops of my fresh water tanks, because those tanks' inlets had to be self-connecting as I raised them up into position under the floor. Make sure your tank's wall thickness matches the grommet - if your tank's too thick you may need to shave down the opening slightly. 3" ABS pipe should fit easily through it, but if it hangs up and tries to dislodge the grommet from the opening (that happenend with me, twice
) some imaginative folk recommend a water-based non-toxic readily-available biodegradable lubricant, i.e. KY Jelly! It's also a good idea to chamfer the bottom of the poo pipe so it will slip through better.