Well I have too ideas. (my avatar is there for a reason)
1, is go buy something called "magic plastic"
they are strips of plastic that are sold in hobby stores...
If it's heated (about 100 degrees F or so) it gets soft and pliable-
it makes sense that the hotter it gets the softer it would get.
Just cover it with the plastic, and dip it in some cold water to make it harden instantly.
You can then point a hair-dryer on high heat onto it to soften it and smooth it out with your hands.
They sell it in a wide variety of colors, I'd reccomend getting some brown, black, and dark green and mizing them together.
If it was me, I'd heat them all in super hot water seperately, mold them into "snakes" and wrap them around the stump in coils, alternating. Then use the blowdryer or heat source to soften them, and smear the colors around with your fingers.
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Second idea, is instead of looking for paint for decor, look for paint made for eating utensils. (I know they make paints for decorating dinner plates)
If it's dishwasher safe, oven/microwave safe, and okay to eat off of, it's probably more than safe enough for your fishies.
(just look at aquarium silicone- it's also the only FDA approved sealant for food contact)
Actually, there's another idea. Paint it, and then cover it with a thin coat of aquarium silicon. two thin coats and you theorectically could use any paint you want, or as an added insurance for the utensil paint.
(They might sell colored aquarium silicone- there's another idea.)
Since you've helped me a LOT, maybe I can help you now.
Here are some links-
http://www.ronjoseph.com/Q&A/B1201_f...y_coatings.htm
Many of the web sites he links to have a "contact us" link, and I know Carboline has an option to ask for a product reccomendation.
As far as minimum Qty, I'm not sure... Could always paint your kitchen sink with it afterwards. lol.
This seems to be a good discussion on aquarium paints-
http://forums.gardenweb.com/forums/l...001912569.html
And someone describing how they "make" aquarium decor... included painting and sealing.
http://www.csd.net/~cgadd/aqua/46g_construct.htm
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Just got another idea that might be the best...
Just cover it in aquarium silicone and roll it around in some colored aquarium sand/gravel.
Would give it a nice "textured" look.
Whatever you do, If you have an extra aquarium filter, I'd submerse your work of art underwater with the filter running and loaded up with fresh activated carbon.
(in a fishless tank) Just to be on the safe side. Probably no need though. You can trust everything you read on the internet. ;)