Not the most experienced in Latin, so this may seem redundant to most, but I'm trying to figure out how to say "to feed on (something)". I'm assuming I just change the case of the object with the verb alo/alere, but I'm not 100% sure which case to use and if i should be using a preposition. Thanks for the help!
Indeed the passive voice of alo with the abl. case can mean what you are aiming for. in L&S:
Hence in pass. with the abl. = vesci, to be nourished or sustained with or by something, to live or feed upon.
It gives classical examples, and this example from the Vulgate (Ex. 16:35):
"... hoc cibo aliti sunt ..."
The 4th dec. noun victus means nourishment, provision, diet, etc. and can be adapted to convey the meaning of "feed on".
The medieval story Navigatio Sancti Brendani (The voyage of st. Brendanus) is fond of using this word, and we find there:
Nonagenarius enim sum in hac insula, et triginta annis in victu piscium, et sexaginta in victu illius fontis (I'm 90 on this island, 30 years I feed on fish, 60 on that fountain.)
Above the suggestion is sum in victu + (gen.) to mean I feed on something. For now I could not attest this usage in classical sources, but we read in Caesar:
maiorque pars eōrum vīctūs in lacte, cāseō, carne cōnsistit. (and a greater part of their diet consists of milk, cheese and meat.)
Here consisto applies to pars, but I can't see a reason to reject a general usage: victus (alicuius) in (+abl.) consistit.