A pretty little guy..
Too strong of a filter can cause stress..
Also keep in mind when you purchase a betta, especially one from a store, that their muscles won't be built up a whole lot right off the bat - from the breeders they are placed in cups (usually around the 32oz size) as young as 5 weeks old, possibly younger even depending upon temperament. So they are in a small container for a while as they grow, then placed into the cups and are in there during shipping and for who knows how long until purchased. If they are from what I would consider mass breeders then their cups could be smaller than the standard juvie cup they are separated in.
So after spending a couple months or so in a small container they are then placed into their new home which is bigger than what they are used to. Even a gallon is a large size for them.. so often times they feel the heavy drag on their fins and will end up chewing off some of the fins in order to swim a bit more comfortably.
Add in a filter with a strong current then they do have more problems swimming with larger fins.
He may need time to strengthen his fins.. you can help by baffling the filter, or just take it out as a 1 gallon doesn't really need it and just make the 100% water change per week instead.
Biting fins could mean stress as well.. If it does have to do with tank size, it's because he isn't used to something larger.. they don't know the difference between a 1g and a 5g if they had spent half of their life in a deli cup - so don't need to stress or worry that he isn't in a larger tank right now - a 1g is a good home for a betta.
Either way, the only thing you can really do is either try to baffle the filter (with an aquarium sponge placed into the outflow section, held in by a new rubber band if needed) or lower the setting.. or remove it all together.
Another thing to try is to make sure he has soft plants to sleep on/in.. also a cave makes some feel secure. For that tank I would get a couple smaller silk plants and a small cave/decoration and see how he does, along with the change to the filter.
Either way, he will still need time to adjust to his new home.. so just keep an eye on him and see if you can find the things he prefers :)
Edit: Just saw the filter.. is it an undergravel one? If so, you can buy valves for the air hoses and then be able to adjust the flow of air/bubbles into the tank. Unfortunately with those I believe there will always be some agitation, which is fine.. it's not pushing the water as it's air bubbles.. so possibly he just may not like the air bubbles.. who knows, they can be so picky at times!
Last edited by Myates; 11-29-2012 at 06:59 PM.