Post pictures of your beloved aquaria here. Also good for pictures of your fish rooms or equipment discussions. If you are posting pictures of identified catfish, please do so in the appropriate husbandry and reproduction forum above.
You could make one fantastic looking shrimp tank out of a 5gal. DIY CO2 is really only efficient in aquaria less then 20gal in size anyway so I wouldnt expect that to cause you any trouble. Before you do that though, make sure you have enough lighting...low lighting combined with high CO2 is asking for algal blooms.
I used to make mine with a one liter soda bottle and about 3/4 of a liter of water mixed with several teaspoons of sugar and 1/3 of a packet of dry baker's yeast from the grocery store. That lasted me about 3 weeks every time I made it and only took about 5 minutes to make. That size tank is perfect for using crypts and small grasses or even glosso depending on your lighting.
As for fish, there are a surprising amount of options here. Dwarf cories (any of the three species) as well as several tetra species such as Hyphessobrycon amandae would be right at home. Look into Celestial Pearl Danios and some other small rasboras as well as killifish.
Low light non-CO2 tank is what I would do. I tried DIY CO2 for quite a while and the fluctuating CO2 levels always resulted in algae. If you're not going to use pressurized, just go lower light with mosses and ferns or anubias and dose Glute or nothing and you'll breed plenty of shrimp. I have a 20 gallon that's breeding Cherry shrimp by the hundreds and houses 3x L10a and 3 long finned Calico Bristlenose, along with growing fistfuls of Subwassertang and Anubias and even small Blyxa japonica. All with about 7 w of LED lighting from a Beamswork LED light strip.