BTW I do notice some green algae on the diffuser. You can use a solution of bleach and water (cap full to half cup of water) or 100% bleach, or hydrogen peroxide (H2O2) (peroxide). Follow the steps below for each method.
If you use bleach and water solution you may need to let it soak for a couple hours.
If you use pure bleach then you only need to soak for 15 to 30 mins.
After bleaching, rinse the diffuser and shake or push the residual solution out of the inside of the diffuser bowl. Then rinse with water. Now with about a 1/4 to 1/2 cup of water and a capful of Seachem Prime (or favorite dechlorinator as long as it can take care of chlorine and chloramine) soak the diffuser in the solution for a half hour or longer if you want just to be safe. After this the last step is rinse and shake and now reinstall. Do this process again when you notice the CO2 bubbles to be too large, or not coming out at 30 psi, or inconsistent or just growing algae. A clean diffuser works best.
For hydrogen peroxide (H2O2)
Using h202 will require a different process. You can do a fast way or slow way.
Using Naekuh's method
This is what i do, and it only takes 5 minutes TOPS, and u get a spanking clear diffuser.
1. I get a syringe... (if u dont have one, u should get one... they are awesome when dosing or spot treating if u ever need it...)
2. I attach a little tubing to the syringe and diffuser directly.
3. I pour a little H2O2 on the ceramic disk, and then i use the syringe to apply vacume so i pull the H2O2 though the disk.
4. I pull the syringe... fill it with H2O2... reattach it to the hose.. and then apply pressure to force H2O2 back though the diffuser onto a clean napkin to absorb the H2O2.
5. I repeat the vacuum and pressure with H2O2 until i notice its easier to feed the H2O2 into the diffuser.
6. Then once im done with the H2O2, i repeat a couple more times with Distilled H2O to force any left over H2O2 out.
Then my diffuser is about brand spanking good as new in less then the time it would take for you to soak it in anything.
(originan post here
Using slow method
Simply let it soak for an hour or more and rinse and reinstall.
Other chemicals you can use
Tilex (Tom Barr uses this see here
Excel, although this can be expensive.
Always remember to rinse very well before reinstalling the diffuser.
A previous discusion about cleaning ceramic diffusers can be read here