As suggested, you need to increase your water changes and cycle the tank... And cories are schooling fish so once your tank is cycled properly, you should get 1 or 2 more...
Otherwise, I do not like freeze dried foods at all... They are not as nutritious and can cause some really bad cases of bloat when overfed. IMO they do not add variety to a diet since they don't have anything good in them... Plus, your betta would much prefer a frozen treat to a freeze dried treat. If you are going to feed freeze dried, make sure you soak them thoroughly before feeding so there is less chance that your betta gets impacted... The general consensus on this forum seems to lean towards very limited use of freeze dried foods but some people do like them.