You don't need a sponge at the output of the filter. The water flow velocity should be high enough to prevent fish from swimming into the output tube. A sponge at the inlet tube would be good, but why not just use a powerhead with a sponge filter on it, and not the cannister filter? The plants provide most of the filtration needed in a planted tank, so all you need to provide is a way to clear the water of suspended solids, and a sponge does that well. This works pretty well on a 30 gallon tank.

 

Yes, you can put a sponge over your intake and then cut little hole for your CO2 line to go into the intake strainer. The flow should remains constant but will slowly decline over a two week period in which you will have to clean out the sponge that's covering your intake.

To enhance using your filter as a reactor, using a small limewood diffuser to break up the bulbs as they enter the intake will help the dissolution of the gas. Even still you'll notice the filter will "burp" out some of the gas that builds up in the canister filter. You can slowly reduce your CO2 output to prevent excess burping.

Also, you may be able to get away leaving the intake uncovered since the strainer is pretty good at sucking in fish (depending on fish size). A couple of rubber bands to form a grill over the intake will prevent fish from entering also.

-John N.

 

I have the 2213 on my 29 gallon and use it as a CO2 diffusor too, I just drilled a small hole in the existing strainer over the intake tube and slid the CO2 tubing up into the intake. A few observations though:

• With 2 2L DIY CO2 bottles, the filter cannot keep up and every few hours will make a loud whooshing noise as tons of CO2 is chopped up by the impeller. Hopefully this is not damaging to the filter! Time will tell...but no way to adjust DIY CO2.
• The intake strainer is small enough that only newly hatched fry would be sucked up, and even then, the 2213 does not have a ton of suction (my guess is it only pumps around 80-100gph once "broken in" a.k.a slightly dirty). So unless this tank is a breeder tank, I wouldn't worry about a sponge, which will be a big hassle removing every 2 weeks, not to mention potentially damaging your filter. (more on this below)
• If your tank is heavily stocked (as is mine) a 2213 is not going to cut it. Mulm is suffocating my hairgrass on the opposite side of the tank as the filter output as the filter just doesn't have enough flow to remove all the solid waste.

Having had a shrimp tank (RIP Cherries), I have to agree with Hoppy partially. Having anything with a sponge over the intake, like a filter or powerhead is a real pain in the butt though. Even in my shrimp only tank, the sponge was clogged every other week, reducing flow to a trickle. This is really stressful on your filter motor and can cause it to burn out and overheat (admittedly much cheaper to replace if its just a powerhead). If you are concerned about little babies and shrimp being sucked up, use a sponge filter driven by an air pump instead. This is plenty of filtration, even for a tank without the filtering capacity of plants and there is no danger of suction. It even provides a source of food for babies. If you, like me, are worried about the ugliness of a sponge filter, try the Jebo sponge filters, which mount on the side of the tank and are like pillars instead of huge doughnuts that take up valuable real estate. They are not the most CO2 friendly filters though due to surface agitation.