Diatom, commonly called brown algae, is absolutely normal in new tanks. Within the first 3-4 months almost all new tanks have it. I just deal with it; use your fingers to remove it from plant leaves (to prevent the plants dying from inability to photosynthesize and assimilate nutrients, etc. through the leaves). Scrape it off the tank glass, or leave it on the side/back glass, up to you. It will be gone eventually. Lighting has little impact on it, in fact, low lighting can cause it in established tanks, but in this case it is there because this is a new setup, nothing more.
Otos and most "algae" eating fish will eat diatoms, but I do not recommend buying specific fish just for this. When the diatom is gone, you are left with a fish you may not otherwise want, and in a small 5g tank that is causing another critical issue in the bioload. This is not the answer. Shrimp eat some types of algae; so do snails but that is minimal.
You have a lot of light for this tank, and you will have green algae problems like you never dreamed of if things are not balanced. I can offer suggestions on balance, but I need to know exactly what you are running--type of light (CF bulbs or fluorescent tube, and what sort of light), CO2, nutrients (fertilizers), what plants, what fish. The latter two are covered in your first post, the plants and fish. You mentioned CO2, do you have a CO2 diffusion system? With those plants you do not need CO2 and I do not recommend it, it is not going to be easy to balance it in a 5g tank. Let us know about ferts and the light, I'll have more then.
A comment on the raphael catfish; yes you realize he needs more room. But the problem is that potentially-large fish that grow (sort of) in small quarters often (usually) develop internal organ problems called stunting. A fish grows all its life, unlike us. It needs adequate space around it from day one, and this involves not just the size of area but the water conditions which are even more critical for growth. Where I'm going with this, is that this catfish needs to move into larger quarters sooner rather than later.