I'd go for a single male betta in that tank. They are very colourful and easy to look after. I wouldn't put any more than that in, 45 litres is pretty small and the filters on Biorb tanks aren't up to heavy stocking.
The Biorb instructions are a bit funny, they say that you need to add stuff that you don't need to add. If the filter is similar to the original biorb then you've probably got a little basket bit with some black and white granules in. You don't need those. If there's a sponge then the instructions might say to throw it out at every water change/every month. Don't do that, just rinse it in a little bit of water taken from the tank. A normal filter sponge only needs replacing when it starts losing its shape.
You'll need to use a dechlorinator, I think biorb make their own but you could just as easily use a different brand which might work out cheaper.
Eaurouge ,thanks for the advice it is a biorb life 45 l tank. What kind of tropical fish can we have and how many, i know its not a large tank and wouldn't want to over fill it. We paid 80 quid for it which we didnt think was too bad.
It takes around 4-6 weeks to cycle a tank (details here) A long time, I know, but it gives you plenty of time to move plants and rocks around until you are happy and to plan what fish you want to keep in there.
Tropical fish are no harder to keep than coldwater, the trick is to choose the right fish for the tank size you have. If you've got something like a 60 litre then a small shoal of one species is best. If you've got something bigger then you could have a couple of different species. Anything smaller than 45 litres and you're restricted to invertebrates really. How big is your tank? If you don't know the volume just tell us the dimensions and I'll figure it out for you.
You'll need a water testing kit to monitor the cycle and see what kind of water you have. Liquid test kits are more expensive to buy but they last longer, so cheaper in the long run, and they tend to be more accurate too (I've always hated the dipstick tests)- the API mini master test kit is a popular one, or the Hagen mini master one. You need to be able to test for ammonia, nitrIte, nitrAte, pH and hardness.
If you've got very alkaline or very acidic water then that will limit your fish choices a bit, but there will be something to suit every type of water.
Hope that helps and hasn't overwhelmed you with too much info all at once! Keeping fish is brilliant and easy enough once you've set it all up, it's just the initial preparation that's a bit of a faff. But it pays to get it right from the start, you'll avoid all kinds of hassle that way.
Really exited just won a fish tank on ebay for dd. Really simple advice please, how long do we cycle the tank for and are tropical easier than coldwater. It does have a heater with it so we can choose either.Please any advice at all would be appriciated.