30 gallons would be ok for the sargassum alone - the dwarf is going to get to be around 6-7 inches...he's going to need a bigger tank eventually.
Issue is, the angler (Sargassum) can't be in a tank with anything smaller than itself - and since they're such slow eaters, anything larger than itself would cause the angler to starve...which is why people with anglers keep them in species only tanks.
You can't have any invertebrates with either of those fish....
so....either you take BOTH the fish back and do a good amount of research BEFORE you buy your next fish, or you return the dwarf lion and keep the angler by hmself.
Personally, I would go with the angler. They are hella cool.
If you do decide to keep him, a skimmer is a great idea, b/c you can't have inverts in there to help with cleanup.
Let us know what you decide - and remember, it's better for you AND your fish if you research them, and the fish they are compatible with, well before you buy them.