right now I run the hagen bubble ladder. I like it because it requires no pressure, past the pressure needed to travel 8 inches under water. I have a few ideas of how to make a better than hagen bubble ladder, but I want to try them before spouting.
I have 2 of the glass "pollen type" diffusers, but not ADA ones, chinese knock offs from ebay. One of them worked once for 3 or 4 hours, with 2 brand new 2 liter setups kicking off at once... but after that, it didn't matter what I did, they didn't want to push. I could even squeeze the soda bottles... the pressure was enormous. they really require compressed gas.
I've also tried both small green air stones and the long black bar style air stones. both work ok, but the slime buildup and lack of visual indication of how much co2 is pumping make them a poor choice without a bubble counter. Even then, the bubbles make their way to the surface quickly. I tried catching the bubbles under driftwood for more absorbtion, but never saw pearling until I switched to the ladder.
The thing with a small tank is that you don't need any more co2 than the yeast provides, you just need it all. the trick is disolving as much as possible at low/no pressure...
I'm curious to see what the bubble counter in between the DIY tank and ladder/diffuser would do to reduce gunk/slime. Someone said recently that it virtually eliminates it.
I can see why you were discouraged about DIY, you've been through a ton of junk with it. I should probably order a couple more ladders, myself, but I'm having fun tinkering, trying to build a better gas trap with stuff I have at my disposal.