Wow! I really don't have an answer. But you will need to do some water changes ASAP. If you were cycling with ammonia, I'd say leave it the way it is, but since you have fish in it, your level is past toxic level and needs to be lowered immediately. Get it down to 1 if you can.
Angelfish, not literally siphon the gravel. You siphon the gravel's surface, so that while you're taking the water out, you're also vacuuming the gravel and therefore removing all the waste from the bottom. If gravel isn't vacuumed for a long time, all the gathered waste on the bottom will produce high levels nitrate, nitrite, or ammonia - which will either kill your fish or make them sick.
While you're vacuuming your gravel, it is also good to stir the gravel at the same time. Sometimes (especially if one has sand or very fine gravel), if the gravel/sand isn't stirred often enough, dangerous pockets of poisonous gasses will form under it, which - if they're realeased - can kill the fish. Besides, when you're stirring the gravel, you're also releasing any dirt that might be trapped under the gravel.