Act quickly. You have a tuna fish spill on the carpet. The first thing to do is react quickly. Sprinkle kitty litter over the affected area. Kitty litter is an absorbent material that will suck the spill out of the carpet fibers. Do not rub at the stain at this stage, because you will only press the spill deeper into the carpet fibers.
Tackle the stain while it's still wet. After five to 10 minutes, remove the kitty litter, which will have absorbed some of the spill from the carpet surface. Spills are more difficult to deal with once dry, so apply a stain-remover product designed specifically for carpets and gently mop up the stain (trying to avoid pressing the fish oil deeper into the carpet). This should remove the worst of the greasy residue.
Dissolve the stain. Spray a carpet cleaner over the remainder of the spill and let the cleaner dissolve the oil. The cleaner will help neutralize the fishy odor and remove any remaining fluid. Once you vacuum up the carpet cleaner, the stain will have disappeared.