Maybe an ammonia spike. I did a liquid test and all was perfect, but who knows? I got a filter for a 50 gal. tank (200 gallons per hour) to keep things healthier. Does a good filter address an ammonia spike? I test with an ammonia strip & add Prime if I'm getting a high indication. I have the Master liquid test kit that I use every other day, but I use the strip to do a quick screening on a daily basis.
Funny, Petco/petsmart says they'll "test your water for free". I assume they had some high-tech super-accurate space-age testing system. I brought water in after the last cory died. They used off the shelf test strips!!! And the parameters were perfect, as my liquid tests had shown. I feel I know more about bettas from this site in 3 mo than any PetSmartCo employee, IMHO. The employees also pronounce them "bay-tah" rather than the correct "bet-ah". If they were spelled "Beta" that would be "bay-tah". Of course they look at me funny when I say "bet-ah". OK I'm ranting...
4 bettas: Butch, 3-year-old violet male PK EE & 3 girls in a 30 gal sorority. I do not name the girls anymore! 1/2 of them die within a month.
30 gal: 9 Oto cats, 4 Ocellifer cats. Inverts: Cherry shrimp & 2 assasin snails.
10 gal.: Butch, his 5 oto cats, 3 assasin snails, and a few snails the assassins haven't caught.