Thanks for the advice! Yes my tank's probably about 33 gallons. There are a fair few things in there, I've got my betta, two sunset platies, 11 neon tetras, 3 three-line corys (soon to be another two), a couple of assassin snails and my fairly substantial pest snail population, and two african dwarf frogs. I also have amano shrimp but I'm not sure how many... I saw one today, but I put five in and I NEVER see them. I have a bad feeling my betta Diemos has gobbled the rest up, as he is quite aggressive, though I've no proof to suggest he would apart from that.
I do really like the zebra loaches, but I wasn't sure they'd be compatible with a betta, I was worried they'd fin nip. Also, I'm not sure if, if I added say 5 to my tank, I'd be overstocking quite a bit? I have a big massive juwel internal filter which came with the tank, but I'm not sure how efficient it is.