Zucchini/Courgette
Cucumber
Peas (deshelled)
Sweet Potato
Green Beans
Melon/Melon rind
Most fruits degrade really quickly, so don't leave then for any longer than 12hr tops.
Courgette, sweet potato, and the harder veggies are usually good for 24hrs
cucumber around 12 hrs
You don't need to boil red pepper at all- it removes vitamins etc, all my bns eat red/orange/yellow pepper and all they leave is the skin. Give the skin side a good scrub before putting in tank.
You can also leave the skin on courgette but also scrub first.
I find like Twotank that i have to steam yams, sweet potato and butternut squash a little to soften them up otherwise the bns wont really touch them