I think the thing people forget with yeast set ups is that they need 100% commitment, unlike a pressurized set up which you can stick a solenoid on and forget about yeast require constant attention to get good results.
It is fact that they are never going to be as effective as pressurized but you can still get good results at a fraction of the cost of pressurized or even liquid carbon. You just have to have the patience to change bottles frequently and most of all match the flow and lighting to the c02 output. If this is done you should be able to get a stable set up.
On a small 20l tank I think a yeast set up could work well if maintained properly. and you could set it up for next to nothing if you have some bits already. The only real cost would be a nano diffuser.
Yeast is unstable because of the reaction process which builds up over the first few days then slowly starts to die off as the yeast is killed by alcohol. so in effect you get a little bit of co2 on day 1 then loads on day 2 & 3 then after that it starts to get lower and lower.
To make the set up as stable as possible you need to run two 2l bottles and change one every 3 days. doing it like this means you're always at the peak of co2 out put.
You also want to make sure the set up is air tight, any leaks will cause instability.
measuring the ingredients will help as well, don't just guess the amounts.
It's best to use proper c02 airline as it doesn't bulge like silicone air line does, but if you just want to try it out normal air line is ok for a while.
It's also a good idea to silicone the joints, again to prevent air leaks.
use 2 bottles side by side with 1 air line coming from each with a 1 way valve. This means you can change 1 bottle with out losing pressure in the other bottle.
From there use a Y connector to take the 2 lines to 1 and into a bubble counter. Its a good idea to use a bubble counter as the yeast mix produces a lot of froth which can work its way into the air line, the bubble counter will collect any residue and stop it from entering your tank.
From the bubble counter run the line to a nano diffuser in your tank placed under the filter out put.
Now all you need to do is work out a good sugar to yeast mix for your tank. Ensure you have very good flow in the tank and work on lighting.
As long as flow is good and your not running your lights for 12 hours a day with a siesta you shouldn't have any algae problems.
Like Jack says DIY yeast set ups can be hit and miss, it might still be a better option to use carbonated water. But if you have the patience to maintain a yeast set up its worth a go, and if your any thing like me its fun to make
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oh should also add you cant turn a yeast mix on and off like you can with pressurized so you have to keep an eye on the fish in case you gas them out. once you work out a good sugar/yeast ratio they should be fine. keep an eye on your drop checker.