1. You can keep some nano fish species in a 5g, but most pet stores do not sell these.. the most common fish you can place in a 5g is a betta splendid (crowntail, veiltail, halfmoon, plakat, elephant ear, etc etc), it is a tropical fish and needs a heater, its also much happier and healthier with a filter (with weak flow or a sponge filter). Do not buy a giant or king betta and put it in a 5g tank, they need a 10g or larger as they are big bettas with short fins that need a lot of swim room.
Contact msjinkzd (forum member name and website name) to ask about nano fish species for a 5g. She sells a lot of them and is very helpful. Be specific about pH, temp, filtration, and plant density/speed of growth (lightly planted with slow growers for example) of the tank you want to put the fish in so she can narrow the list of possibilities to fish that can live in that environment.
Bettas and shrimp are an iffy combination, some bettas will eat the dwarf species (Cherry), and even killing the larger ones (amano and ghost). Any fish that can open its mouth wide enough will eat dwarf shrimp, if it can't eat the adults it will eat the babies (shrimplets). I would not recommend shrimp for a newly set up tank, they need a filtered, cycled and well ages (2-4 months after cycled and planted before adding) tank to ensure the growth of microfauna that they forage for constantly. If you are an expert at keeping shrimp and nano tanks shrimp can be added sooner (especially if transitioning moss/other plants from a tank you have already aged and cycled). But its not recommended for someone new.
2. If you're not proficient in keeping aquatic plants then yes filter is a strongly recommended
addition. If you are good with plants look into the Walstad method
, it uses soil based tanks and large fast growing plant mass to absorb ammonia. Do not set up a sparingly planted tank with slow growers and no filter and expect ammonia generated by waste/leftover food/plant decomp to be absorbed to make readings 0ppm. If you want a filter-less tank a larger tank is better to dilute ammonia more (under stock fish and over stock plant).
3. No experience with enriched substrates like aquasoil, ada maazonia, and the like nor have i tried fluorite. I use either Black Diamond "sand" (tractor supply co 20/40 or -60 grit-$8 for a 50 lb bag) dosed with liquid Seachem ferts, or with osmocote+ root tabs placed deep in the sand (1 per every 3" square area with root feeders like crypts, vals, swords, bulb plants.. NOT
or under hardscape/decor-add new tab once every 6-9 months). OR I sue Miraclegro Organic Potting Mix (MGOPM) capped with black diamond (Walstad method) with no other ferts used (except maybe Seachem Excel). The ideal substrate depends on what plants you intend to use. I personally only think the enriched substrate like auqasoil are good for short rooted growers like dwarf baby tears(hc). If you want to plant and add fish immediately I would not
use soil or enriched substrates as they can leech ammonia for a while (2-6+ weeks depending on amount used and plant mass/speed of growth).
4. Not every light works for plants, if you want to use a cfl try to get a 9-13 watt 6500k (kelvin) ideally. But anything from 6200-6700k works well. Use a lamp with a reflector. I think a t5 might be a bit expensive for a short tank, would recommend for 3'-4'+ long tanks. If you want cheap lighting: home depot 4 pack 6500k 13 watt cfl[click]
, clamp on lamp reflector[click]
less than $15. 1 lamp should work of the tank. You can clamp to a shelf above the tank or get some cheap chain to hang it.-I did this with my 7g, 6g, and 20g long.
5. Anubias, anacharis, hornwort, and riccia also work in low light tanks. Anubias and ferns are rhizome based plants, the rhizome if the horizontal part that both roots and leaves grow from DO NOT BURY THE RHIZOME in the substrate, ever! It will kill the plant. Instead die anubias/fern to decor/wood/rock or a lead free plant weight (or even glass bead).