Mild cases of fin rot, I treat with aquarium salt 1tsp/gal along with 100% daily water changes for 10 days, I also like to keep the water temp in the more room temp area of 76F, most bacteria thrive in warmer water and I like to pre-mix my salt water in a 1g jug to make dosage easier and keep the jug and QT container with the Betta in the same location to keep the water temp fairly close.
You can also add IAL or oak leave to the jug for added benefits.
Feed a high protein diet to promote healing
My home made fish food is a general food for adults more than fry, however, I do start feeding it to the fry along with live foods at about 8-10 weeks depending on their size.
I make large batched and freeze it and use as needed
this is a smaller batch: all human grade foods used
1-fresh raw large shrimp-chopped
4-5-fresh baby spinach leave blanched
2-3 fresh crushed garlic cloves
I mix/smash all this together to make a green, garlicky-shrimpy paste, I then place it in a plastic baggy and flatten it and freeze, breaking off what I need each day to feed.
This is approx amounts and leaf and clove size vary.
Missing ventral are usually related to bacteria on the bottom of the tank and has been linked to the bacteria made from decaying microworms.
Keeping the bottom of the tank clean with frequent vacuuming and using common snails to help with clean-up can help.
Oldfishlady, I am keep reading posts about your "home-made fry food", and I am dying to know what it is lol. Unless it is a secret recipe..
I know that my first/only spawn has some fry without ventrals, and I've heard it is from malnutrition, and not genetics, so I hope to improve the diet of my next spawn. Seeing as you are very experienced along the lines of breeding Betta Fish, I figured that asking you was best :)
Fresh dechlorinated water IMO/E is one of the best things for fish long term health.
Making regular water changes of at least 50% twice weekly in 6g and less aquariums with filters and 50% and 100% weekly on unfiltered tanks that have one Betta, more depending on stocking.
10g and larger at least 50% weekly depending on stocking, filtration and live plants use.
This is with uneaten food removed after feeding and not overfeeding