Hi,
repeatedly used petroleum ether acquires a pungent smell of fish blood and turns yellow, if you use this petroleum ether again, the crystals become very yellow, the question is, can petroleum ether be purified by simple means or is it already a waste product?

It could be purified by distillation. That's not very complicated but requires some equipment to do safely.

You can remove some of the contaminants by washing it eg. with dilute sodium carbonate. If you don't care about keeping any alkaloids that may still be in there, alternatingly washing with alkaline and acidic solutions might be even better. You can probably do this if you have a bottle and a pipette, but a sep funnel comes in really handy

Given the fishy smell, it's probably best to do an acid wash whether or not you plan to distill the solvent as well.

+1 for using any lab equipment too.