Never ask a pet shop employee for fish advice - -they are clueless 99.9% of the time.
If you don't use conditioner, the fish will most likely die. There is stuff in the water like chlorine, heavy metals, ect that are deadly to fish.
Also, even with a filter, you need to do weekly water changes. I don't think a 2.5 can cycle but if you use a filter you should be aware of the nitrogen cycle as it too can be deadly for fish.
EDIT: since you just got him, it may take a few days for him to settle in. Since the tank is kinda tall, you may want to get a tall silk plant with lots of leaves for him to rest on near the surface. I would suggest Indian Almond Leaves but you can only get them via sellers in Asia. They release tannins into the water and when i first put the leaves in, I usually find the betta chillin under the leaf. After it gets a bit more water logged, the leaf sits lower in the water and I usually find them resting on top of the leaf. They need to be taken out after 2-3 weeks so it does not rot and pollute the water. They also tint the water amber colored- like a bettas natural habitat
Last edited by Tikibirds; 04-22-2012 at 12:02 AM.