I think my tank is now cycled because my nitrite and nitrate levels are levelled off and I have my tank 6 weeks. Unfortunately, while I was cycling it two fishes died (I only had one in there at a time). I was a little bit naive at the start and didn't really understand the cycle so that's why the poor little first guy died, but I'm totally confused at the death of the second little fella. He'd been going drastically downhill for about a week and gave up yesterday but I think that maybe the nitrite and nitrate levels were still too much for him when he went in to begin with.

So my question is, when you cycle a tank, should you expect casuaties?

(The other thing is that I have a biorb ... which I am getting rid of pronto and getting myself a proper tank with a proper filtration system which some very nice people on this site made me aware of!)

a lot of casualties happen as a result of cycling, but the way it is minimised is by keeping daily checks on the water params (ammonia, nitrite and nitrate) and keeping up daily water changes to keep the levels at an acceptable level so they do not harm the fish.
Obviously there needs to be ammonia and nirite in the tank for there to eventually be a presence of nitrate, but these levels need to be kept low so they dont adversely affect the fish.

I'm sorry you lost your fish to cycling, it does take some time to get it just right. I'm really sorry for your loss.

Most biorbs I've seen aren't all that big, the largest I've seen is 5 gallons which would be lovely for a betta.

Kat has some good advice, regular water changes will keep the ammonia/nitrIte levels low. It is much easier to cycle a tank if you use test kits to monitor the ammonia/nitrIte levels, then it's not guesswork on how much water to change.

Another tip is to use beneficial bacteria, such as "Cycle" or "Stability" which in my experience will promote nitrogenous bacterial growth, shorten the cycle and keep ammonia/nitrIte levels lower.

Also you can use the water conditioner "Prime" which de-toxifies ammonia/nitrIte making it safe for fish while it is still available in the water for the nitrogen cycle.