unfortunately there are not a lot of options for predators in a 10g.
your best bet is probably one of the dwarf species of pufferfish. there are a few out there that will work in a 10g tank.
another option, and my personal favorite, would be exodon paradoxus. (bucktooth tetras). however, they need to be in a group of 6 or more, and 6 in a 10g is pusing the limits of general stocking rules. in order to pull this off you will need excessive filtration and you will need to upkeep a regular WC schedule.
A 10 gal is also to small for a pair of rams in the long run as its harder to keep the water quality up to par for them.
There are a few killies that can be territorial and a 10 gal makes a perfect tank for them. Just make sure you have a tight fitting lid as they are avid jumpers. A small group of pea puffers would look cool in there also.
Mosquitofish!! they look like wild type fem,ale Guppies but they are absolutely the most aggressive fish i have ever owned...i once had one try to attack a Cobra Guppy...that was in the tank next to it! it went after a fish that was 2 glass panes away...lol
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