you could always get a snail
I wouldn't get snails lest you know what will happened. Goldfish eating a lot also gives the snails a chance to explode their population.
This situation worsens as snails are rather hard to remove.
If I were you, stick with Malayan Trumpet Snails.
Avoid the ramshorn and common pond snails. Apple snails are also a good choice.
Malayan Trumpet Snails can eat detritus and dig in the substrate so this also prevents anaerobic spots if you are using sand.
Would a goldfish live longer if there is a filter in his tank or bowl?
For a fishbowl, only bettas are the good choice.
My advice on feeding is feed sparingly and vacuum the leftovers. Make sure they consume the food in 5 minutes. Vary the food menu as the fish gets different nutrients at that. Don't feed them on only one food. It's very useless as they never get to receive different nutrients.
What's more is that aritifical foods tend to have less vitamins in it. You might try the mosquito larva, daphnia, bloodworms and krills.
I'd go with frozen rather than live ones to reduce the chances of introducing pathogens.