What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
Definitely soft water. PH is currently 6.8
My mother got an anglefish and he attacks the other fish - too aggressive for me and I don't want to risk upsetting a happy tank. For the same reason i don't want to risk betta and i hear barbs are aggressive as well. Rasbora could work.
I have built my tank up slowly - if I added another 6 danio and 8 tetra in one go do you think i'll get problems with bacteria - should i top up bottled bacteria? Tank is a few months old and has been ok so far.
I do have plants but 5 are new and I don't know if they will do well, previously i had plants that died. My fish sometimes get stressed if I put
the light on and i put them first. My new plants are slow growers and may do better with less light.
I wanted floating plants because i was told it will make the tetra feel better, but my local store has been out of stock. I will try amazon, as i understand there is a good seller.
There a few on-line shops which sell water sprite. The seller k2aqua on eBay usually has 3 varieties - thalicroides (the one everyone else sells), cornuta and fein. I've just looked and unfortunately all three are out of stock at the moment.UK members may help too, I believe @Essjay has acquired Water Sprite from some local store.