I have my lovely male betta in his tank, and recently got 10 neon tetras to keep him a bit of company. There's been no problems at all between them, and I can't decide what to get next. I have a 50L tank, weasling my hubby around to a cheap 200L that I saw the other day, not 100% sure yet as to whether I'm going to move my boy and his friends over to the 200L, or start a different community though.
The lady at the pet shop suggested either cat fish or cherry barbs... I like the look of both, but don't want to crowd my tank.
I have a heater, filter, air, and light setup with both real and fake plants (slowly replacing the fake with real), and just wanted to see what everyone would suggest.
If I did my math correctly, you have about a 10gallon tank...if that is the case, your tank is already full based on 1inch per gallon "rule". If you add more fish, it would be too much bioload for your filter, resulting in a messy tank and too much ammonia. You could probably get away with adding some snails but anything else would make your tank sick and too many friends in such a small space would probably stress out your betta.
If you went for the 200L tank, I would get a school of at least 6 Cory cats(bronze, panda, sterbai are all cute)...they are so much fun to watch!
Of course, it's kind of the pot calling the kettle black as I usually overstock my tanks, too!
The 1 inch per gallon is pretty outdated now. You should base it on bioload, the space needed for activity (neons are very active) and compatability with the species you already own. Plus you need to take into account if you have plants for hiding spots and appropriate substrate for bottom feeders like corys.
I did some rough calculations on AQadvisor and if it was 52ltr it would be 14gal. AQ says it's 76% stocked but I'm not sure on the method they use. Corys would work well. I have 4 bronze and they are lovely little fish and are highly entertaining, something smaller like a pygmy or panda cories would be lovely and will be easier on the bio load. You'd need at least 6 and they prefer sand substrate to protect their barbels. Very fine and smooth gravel is adequate but sand is much more rewarding as it's so entertaining to watch them sifting away searching for food :) Avoid anymore fast moving schooling fish as they may stress your betta out if there's too many schools and they would max out your stocking, I only let my stocking get to 80-90% so if I get pest snails it wont cause too much problems with my water quality.
The 1 inch rule is still used. Although I do agree that it's outdated and basing it on bioload/activity levels/ hiding places is a much better and more thorough method, but in the interest of keeping answer short and easy to comprehend, I figured more people would get the 1 inch visualization quicker.
I'm on my htc droid so, it is all done in my head....if the tank is 14g, I would plant it moderately so the fish can carve out their territories, leave enough room for your schools to play, and I would get corie catfish (i happen to love sterbai Cory cats...but whatever your taste!)...cories are great fish, playful, and they look like an intricate dance routine.
With your size tank, I would get at least 6 corie cats, since they will do better in a school.
Also, keep in mind that this stocking of the tank takes care of all 3 levels w/o overcrowding one level: bottom feeders, neons are mid water column, and betta at the top.
Just beware that your betta would probably dislike too rambunctious roomies as well as overcrowding and might begin to get violent.
The one inch per gallon rule is actually valid here. It breaks down with fish larger than 2 inches because fish grow in 3 dimensions not just one. But that is beside the point. About the stocking: personally, I wouldn't add any more schooling fish to the 50L (13.21 US gal). I believe a single bottom-dweller like a bristlenose pleco (more info here) could work and would help a bit with algae cleanup, but you still need to feed them. Depending on how old your tank is, you could also do a few otos (read more here).
As for the tank looking rather bare, I would plant it more densely. Both the betta and the neon tetra will benefit from more cover as these fish naturally come from pretty densely planted streams. They will color up nicely with some floating plants, too.
If you get the 200L (52.83 US gal) then you could have many more options that include a school of cherry barb or cory cats. I suggest you look through the fish profiles here and here to get an idea about these fish and find others that might interest you. And finally, to begin building a stocking plan, we need to know your local water parameters such as pH and water hardness.
So if I use the 1 inch per gallon rule I can get a fan tail in my 5 gallon... That is why it doesn't really work. Go by the fishes swimming levels, activeness, bioload, and aggression. You don't want a large amount of cories in a tall tank as the floor swimming space will be cramped.
I would say cherry barbs, cories, harliquin (spelled that wrong lol) rasboras, and tummy nose tetras could all work. Rummynoses are harder to find as they aren't super popular, though because of their nice coloring and schooling tightly (They actually school together, unlike neons and cardinals that scatter the tank) they are becoming more popular. Cherry barbs are also very hardy, some say as hard as bettas are.
I avoid the inch per gallon rule. more like pooper vs not-as-much-pooper :) hence why bettas can live in say...2 gallons... VS a goldie in a 5 gallon. It really is outdated... good as a standing for some beginners as long as they understand which fish they want have a higher and lower bioload, however I've seen many people in my town say "well inch per gallon... he'd fit in a 5 gallon!" Best bet, look for the fish, the ADULT size of the fish, and bioload. :) If you do have a 2 inch tetra (who generally have notso-bad bioloads), 5 would be best in a 10, then you can use the inch per gallon rule to HELP, but not STEER
Good point koimaiden and kfryman...keep in mind the footprint of your tank when choosing fish, especially bottom dwellers. If your tank gets its increased gallon/liters by going up, your bottom dwellers will have less space. It might be a better idea to get a pleco...but beware what they tell you in fish store is probably wrong! My local fish store sold me a beautiful pleco stating he would get no larger than 5inches...this was back when I was not researching...he is q14inches and still growing. That's why koimaiden specifically suggested the bristlenose pleco.
So if I use the 1 inch per gallon rule I can get a fan tail in my 5 gallon... That is why it doesn't really work.
Well here is another instance where the 1 inch per gal rule breaks down: deep-bodied fish like fancy goldfish. Some are taller than they are long (my ryukins for example). Oddly enough, with normal shaped fish that will stay about 1 inch long, this rule kinda works. But in general, this rule has too many problems to be considered reliable and should not be recommended to beginners.
There have been some good other suggestions for fishy companions, but I don't think rummynose tetra would be good. There are actually three species on the market that are commonly sold as "rummynose tetra" (Hemigrammus bleheri, H. rhodostomus, and Petitella georgiae), and only one of them is actually suited for the warm temps betta like (they are also seen in discus setups for this reason). The other problem being that they like large tanks. This is one tetra you should really have in schools of 10 or more and they grow around 2 inches long. However, if you have soft enough water, they would be great in your 200L!
Cherry barb and harlequin rasbora are good fits, tho. Just make sure to get at least 6 of each as they are schooling fish.