I'm choosing between a 20L and a 29 for my next tank. I have read a lot of threads that the 20L is the way to go so I was set on that until I researched lighting and am set on a 30 inch Finnex 24/7. threads on if this is too much light for a 20L are not as easy to get a general consensus on and the SE is so new I'm not sure there is a general opinion. Am I better off with a 29 for the depth or will a 20L be ok. I will be using Ferts and CO2.
One of my tanks is a 20L and I have a Finnex Planted + Fugeray which I am told by Finnex is the same lighting capacity as the 24/7 (without the bells and whistles, of course). I have run that light on this tank almost 2 years now.
The one thing I highly recommend you do with a 20 L if you want to go that way is to double filter. I had 2 filters but put them on one end (too lazy to cut the back out on the opposite end of the cover) and had nothing but trouble. Moving it to the opposite end (cut another spot out for the filter near the other corner) made things TONS better. Fish are happier too.
The 20 L takes a bit longer to get over that cloudy phase too.
However, it is more fun to watch the fish swim and once I moved the filter to the opposite corner, things picked up greatly. I have two 50 aquaclear filters (I changed up one of the impellers to the 70gallon impeller)- one on each end of the back and they are full streaming. I also have a bubble stone in the middle. The fish are doing great in there, and although the Planted + light is a bit strong, if you have the 24/7 I bet it would be even better b/c you are not having constant high light on it.
As long as I do weekly partial water changes (25-50%) the tank does very well. My plants are growing great too. My java moss grows really fast too. A bit of BBA but with the shorter light span of the 24/7 at full light, I think that would take care of that too.
I hope that helps a little bit anyway. I have a 29 too and like them both but for the smaller fish I prefer the 20L because they can swim back and forth and it still looks like a nice big tank. My 29 I have rainbow fish who are moving eventually to a bigger tank, but they needed at least the 29 for now.
Of course, it depends on what fish you plan on putting in there as well.