I'm sorry to say, but it looks like it was due more to improper water changes then it was due to illness/discolored waste.
As Tiki mentioned, weekly water changes are a must do for each tank, regardless of size, cycle, live plants, livestock.
The snail alone would create a very large bio load and therefor would of required a bit more water change in that tank alone.
For 5 gallons filtered, weekly 30-50% water change with regular vacuuming of the gravel is needed as a filter does not actually clean the tank, but hides the waste.
The 10 gallons filtered, weekly 25-30% water change with regular vacuuming is needed.
Don't forget the water conditioner. Unless you use a well for your water source, it is highly recommended to use water conditioner as you won't know what exactly is in city tap water. The ammonia build up in the tanks that aren't being properly cleaned, as well as no conditioner is a very high risk of poisoning and death.
75 is a little low for them, as they are tropical fish and require warmth, recommended temp would be 78-80*F- you would be surprised at the difference only a few degrees makes in fish and their health.
It sounds like it may of been SBD problems going on, which as mentioned above can be caused by over feeding, and unfavorable water conditions.
With the weekly water changes, feedings of 2-3 pellets per meal, twice daily and proper heat should keep them healthy.
I know you mentioned doing it for years, bettas are hardy and can live in unfavorable conditions for a while.. but it will always catch up to them, and if you have had problems with death of them or other fish after less then a year then I highly recommend doing the appropriate changes in care for them so they can live a full life, happy and healthy.