Do you think that having a sponge over the output area helps a lot?The bacteria that eats Ammonia is only a small percent of the organisms that make up your biofilter. So imagine supressing it. You will get even less results.
These same bacteria do not like pH below 8.5. In a planted tank most folk try to have a nice low pH by pumping a lot of CO2. At pH=7.0 the bacteria functions at about 10% of their max. At 6.5 forget about it.
Thing is this is not the only living thing in your filter that we call a biofilter. So it's not like you can supress your biofilter to do absolutely nothing. Also you have other areas where these bacteria live - substrate, plants, decorations.
Your biofilter is not really needed in a smoothly running well established platned tank. Remove it and you wil see better plant growth because of the extra Ammonia that is not eaten by the plants. But the filter is an insurance against possible sudden changes. Which happen even if you don't do anything radical.
One other reason to not pump CO2 in the intake is possible accumulation of gasses. Your filter maybe running 90% empty, full of gasses. These gases are not CO2 either.
Damaging to plastic parts - yes, CO2 can make them brittle. How fast is a different question. The plastic of the cheap Chinese big size canister filters on ebay can actually fall apart without help of CO2. Cheap is cheap.
Long story short - get yourself a glass diffuser or if you like ugly get a small powerhead, put a sponge over the outttake, stick the CO2 tubing in the intake and put that thing of beauty inside your tank. Efficiency is amazing and if you have a lot of plants you can hide the ugly a bit.
That's very true Smitty. But the first sentence of my post presupposes the legwork I did on the particular parameters of my question. I then did multiple google searches of "putting things in a canister intake" (as well as variations) and came up with zero results. After all, what exactly would a person put in a filter intake by design aside from co2? I thought you might have something fresh to offer that had previously been undiscovered country.I know personally, if I search something to learn about it, I often find I understand it better as well as finding some other answers to questions I might have.