I'm going to get more fish for the 20 gallon in February and right now I'm trying to figure out what would be the best fish to mix with a male betta. Right now the betta says hi to me whenever I walk up to the tank. Anyway I'm thinking about adding a plecostomus, a few neon tetras and maybe a rainbowfish or two. Also wondering if I should risk buying shrimp or snail.
Well without sounding harsh, it's time to read up on each of those fish! First off, measure the dimensions of your tank. How deep, long and wide is it and keep that in mind when researching fish. A tank too small only creates a situation of overcrowding and high ammonia which will cause fish to die. (Sadly it always ends up being your favorite fish that gets taken).
The Common Pleco grows to 2 foot long so he's out of the question for either of those 2 tanks.
Rainbowfish like groups of 5-7 and require a tank that is 48 inches long... that's a 75 gallon and up. I have 2M/4F in my 75. There is a dwarf variety but I found them harder to keep, more delicate ie harder to keep.
The Betta commonly 99% of the time is housed ALONE. Maybe a snail, shrimps or possibly the Neons.... I've never tried the Neons (with a Betta). (Don't make the mistake of buying a crayfish/lobster type fish... they hunt at night and will likely eat the Betta. I would never add one to my tanks, I value my fish too much. JMO.)
The Betta is a fighter and needs to be housed alone, no other Betta, unless the tank is
securely divided. Many keepers here will divide a 10 gallon tank into 2 or 3 parts and keep 2 to 3 Betta. I give my Betta the whole 10 gallons to himself. They DO require a heater so many keepers make the mistake of putting them in vases & bowls sadly. They are highly social and interact with their owners. Enjoy him!! I dare you not to fall in love!!
Hope this helps. Please do yourself a favor tho... DON'T listen to the local fish store when they tell you YES these fish CAN go together, (they only want to make money). If you see something you want, go home and research it, then decide and go back the next day to get it/them.
We all want you to be successful and enjoy your tanks. But the wrong combination can be disastrous and highly stressful. Go slowly!!
Keep us posted, we're here to help!! You said you've had that 10 gallon 19 years?