I found this gorgeous rock out in the forest and put it in my aquarium. It’s light green with quartz veins through it.
Well, its a rock, like you said. That means, just like you said, its made of at least two different things – quartz and something else. So there’s that state of affairs that you have to deal with on your own.
There are two things that can help you figure out a mineral – that’s one of the things in this rock – from online sources: specific gravity and hardness. You can try to find out how heavy it is for its size, if you have a scale that accurate enough, and you can determine size by how much water it displaces. Or just in general – is this a heavy rock for its size? And you can determine hardness in general terms too – what can it scratch, and what can’t it scratch, what scratches it, etc. They all help narrow down your choices.
I don’t know what kind of rock it is, but I wanted to say that your picture is lovely. 
Thank you!
I started with my (failed!) pickle jar ecosphere, then went to a 2 gallon filtered tank and now I’m sucked right back into the aquarium hobby with a 10 gallon planted tank.
Offhand guess, it looks like it has some serpentine in it.
It’s impossible to say, especially from an underwater photograph I can’t even tell if it’s sedimentary, igneous or metamorphic.
Light-greenish colour like that usually pings metamorphic to me, but I’m not sue if I’m seeing a linear texture running near-vertical, or not.
Can you get a closer photo, out of the water (but still wet)? Especially of the, what looks like, either crystals or clasts in the centre of the rock in that picture. The clear ones, not the big milky quartz one.
It could be granitic, and zooming in doesn’t show any obvious sedimentary layering other than the bands of what look like quartz. The green could be due to amazonite.
I agree, the greenish part looks like serpentine, which is not common overall but is quite common around my area.
Does serpentine form with bands of quartz in it? I don’t know one way or the other, but if you found out “no”, that’d rule it out.
Serpentine is not very hard, and is kind of porous, as I remember it. You can polish it but the surface remains finely pitted.
I think serpentine would foam in vinegar. I know it ages badly from acid rain.
These would all be tests you can perform with the rock, but which we can’t guess from the picture.
Generally no, serpentine+quartz is a pretty unusual combination. If it was serpentine, I’d be more inclined to think the veins were carbonate than quartz. But I don’t think it looks like serpentine, offhand.
The new pictures didn’t turn out very well, sorry. I had already put it back into the tank when I saw that they were a bit fuzzy. I’m also curious how a rock like this is formed?
Kryptonite
In its most well-known form, it is a green, crystalline material originating from Superman’s home world of Krypton
There you have it
It’s a pity this one is blurred, I think it would have been the most useful.
Even blurred, it looks like a metaconglomerate to me - all the clasts look quite rounded, and I’m seeing a mix of quartzes from white to smoky. Or possibly those are volcanic. There’s quite clearly bedding (layering). The top bed (leftmost) has larger pebble clasts, the middle and right has more gravel-sized clasts and more matrix. The (I’m for now going to assume quartz) veining look sub-parallel to the bedding as well. The green-coloured matrix is likely altered clays, probably something like a mix of chlorite and epidotes. I highly doubt any serpentine is there.
Conglomerates can have a wide variety of origins, in vastly different deposition environments ranging from glacial to rivers to deep ocean post-quake sediment flows. Based on your location, possibly submarine fan deposits of the Nanaimo Group, but that’s just a wild stab.
However the conglomerate was formed, it then went through low-grade metamorphism - not particularly high temperature or pressure, since sedimentary features like pebbles were preserved. That caused the alteration of clays and other minerals in the matrix to greenish minerals. Probably at the same time, the veins were emplaced.
Wow! So much awesome information - thank you. Next water change I’ll take it out again and get some better, non-blurry pictures.