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co2 straight into filter inlet

I’ve used this method for the last 18 months, no issues at all and not a single bubble in sight.

The filter purges itself every 12 hours and I can’t say it dumps anymore air (Co2 if you know what I mean) than normal.

I used to have a Fluval 88g diffuser inside the filter intake cage but that got a bit fiddly taking it out for cleaning so did away with it. Never had any issues with filter seals and been running like this for almost three years - I do re-grease (Vaseline) all the seals on every filter clean tho’ so not sure if that helps things.

Saves a few quid on diffusers, no cleaning and one less thing to worry about - makes calibrating Co2 delivery rock solid also, no pressure variations etc.
I've heard it can work well, but I guess it depends on your filter.
I tired it for a while with an Eheim Ecco Pro 300, and got fed up with listening to it cavitate and burb constantly.
It is more effective if you feed the output of the in-tank diffuser into the filter inlet. This will feed smaller bubbles into the filter and will reduce cavitation.
Large bubbles straight from the line is more difficult for the impeller to swallow.
Also, if you can find a pinwheel impeller compatible with your filter this is the best combination.
It's possible to convert your impeller to a pseudo-pinwheel as well to improve bubble mashing.

The big issue for me would be he fact that you can not see what is happing inside the filter, for all you know there might be one huge big bubble of built up gas.
This build up might take an hour or even two hours to disperse after the gas is switched off.
However the method does seem to work for at least half the folk who try it....
It definitely doesn’t work well with some filters, I’m using an FX6.

I have a JBL Cristalprofi 901 on a second tank and thats horrible - spews huge bubbles out and the noise is terrible, a Fluval 406 before this worked very well.

Appreciate whats said regarding a diffuser and the smaller bubbles/mist entering the filter but (and I’ve probably not used the best diffusers being a cheapskate) I always seemed to be fiddling with the needle valve due to inconsistent Co2 output - with this method I never touch it.

If you have a friendly filter it doubles up as a free reactor.
Yes, a lot depends on the pump and impeller design as well as the amount of media in the filter. As I mentioned, using a pinwheel impeller (also referred to as a needle wheel impeller) chops up the bubbles finely and reduces the buildup.

Here is a typical example of a pinwheel. Theses impellers are used by reefers in their protein skimmers and it does a great job of chopping up bubbles into a fine mist. Although saltwater allows a lot finer bubble size than fresh water so it isn't as effective in our application and the penalty is that it lowers the throughput.

Barr's (plantbrain) DIY method by cutting the impeller blades:

When it works, it works better than almost any other method (and is cheap). So it's always worth trying.

Hi John,
As I mentioned, there can be a reduction in throughput, but the benefits are worth it. Pinwheels, in general do not produce as good a throughput as the normal shaped impellers do, but keep in mind that one of the reasons we are attempting to follow the 10X rule is to overcome the inefficiency of dissolving the gas. If we improve the efficiency and get more gas into solution, the flow rate can be lowered without penalty.

Without going too off topic, does the above mod have any effect on filter turnover?
Yes reduces flow considerably. Also most canister filters have the pump post filtration side thus the CO2 has to pass through the filter elements first, which goes a long way to dissolve CO2 even before it gets to the pump head.

Pin wheel and other filter mods work best with CO2 injection into the pump head and then into suitable "reactor" (ie canister with little lumpy media in) to allow the smashed up super fine CO2 mist to dissolve fully before going into the tank.....