Today I bought a bulb assortment from petco that has waterlily, onion plant, barclaya, and aponegeton. With the onion plant is it neccessary to plant the bulb in the gravel or just drop it in the tank like the aponogeton?

They're a pain,but I plant the root parts and leave the base of the Onion Bulb out of the gravel.It takes a while for this plant to dig in ,so most times,my are seen floating though.
I have to watch that though,because Mambo,my Black Moor,loves eating the new roots.

Thanks Devs. I think I'll wait until I start to see roots, then I'll plant them. I dropped them in yesterday and they still float like a cork.

As Devs mentioned, it is important to plant them so that the upper part of the bulb is above the gravel, otherwise the bulb will rot. So, just plant the lower half of the bulb. You can weight them down with a plant anchor around the base of the leaves to help keep them from floating, then once they are rooted, take off the plant anchor.

So it's ok to plant them like you and Devs mentioned even though they're still floating, nothing sprouted yet? Also the bulbs don't seem to have much to them. They "look" normal, but the core of the bulb feels like its small, only a small hard part along the centerline of the bulb. Should the outer layers be loose or tight and compact like the inground onion starts you get at a garden store?

One of the onion plants has started to sprout! It's taken on a green tint, especially near the tip of the bulb. The first leaf has started to poke its way out of the bulb. I couldn't find plant weights at the 2 LFS I usually go to, so I've put it between the bubble wall and the side of the tank. It's tight enough to hold the bulb and keep it from floating, but loose enough that it doesn't pinch the bulb. Would this be ok, at least temporarily until it takes root or I can find some weights?
On a related note, one of my aponogeton and 2 of my lily bulbs are sprouting and so far the guys have left it alone. The other ones haven't been so lucky