The corner box filter sholud not have caused the death of your betta, unless you used a 150psi compressor to drive it. My guess would be that A.) There may have been some type of lethal contaminates in or on the filter and its media, or B.) He was already ill and you may not have realized it. In either case your are now without your fish.
Some fixes for your problems, although they won't help you now they will be of use in the furture. 1.) Clean, with clean, water, no soap or detergents, all things placed into you aquarium, even filters and their media. 2.) If you add a filter, powerhead, power filter or such make sure that it does not overpower the tandk and its inhabitants. If it presents excess current, turn it down, if possible. You can also, as in your case, add plants such as elodea to help break up the current and provide a safe respit for your betta. 3.) keep a close eye on your fish. Take note of how they act. Any deviation, even the most minute, may be a sign of sickness.