Whilst Barbie makes a good point, I'll make some suggestions with regards to what would be suitable to the size of your tank. If we follow the 4L x 2L x 2L, we find that the smallest side of your tank [the width] gives a maximum length of 6.5".
Of the commonly available species, this would give you:
Most Peckoltia species, such as
.
Most, if not all, Hypancistrus species.
for example. Forget about getting
tho', as those are VERY expensive [$150 for babies and $200+ for adults], and not at all easy to get hold of...
Small Panaque species, such as
or
. Note that there are several BIG Panaque species that are completely unsuitable for your tank.
If you follow the links to the Cat-eLog, you'll find a link to the "Genus thumbnails" for all species of that genus, at the banner line at the top of the Cat-eLog page. Where it says "Loricariidae::Panaque Maccus", you click on the Panaque, and you get a list of all Panaque species.
In the genus thumbnails you can look for fish that are pleasing to your eyes, and then make a short-list of ones that you like, and I'm sure someone will be able to supply some further details and suggestions on those particular fish.
In a tank that size, you could quite easily keep 3-5 fish, either of the same specie, or for example two groups of different species.
And please supply your tap-water parameters, as that will help avoiding you buying a fish that won't adopt to your local water.
--
Mats
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