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I have a 135 gallon planted tank with a 14 inch clown knife, a 6 inch green terror and a 6 inch pleco. They've been together for a year and a half. I'd like one more fish in this tank. Would a silver arowana be a good choice to add to this tank? If so, what size should it be? Any other suggestions would also be appreciated.

the common answer u will get is: you shouldnt get the silver arowana.

Your tank is pretty full now, and a silver arowana needs about 300 us gallons.

prehaps try a smaller fish like a teacup ray.

Your tank is pretty big and would maintain an arowana for a good while, but it would be preferable to maintain such a large fish in a larger tank. Once the fish's length matches the width of the tank that fish is too large for it; the adult length of the arowana can be about 3', which, if I'm not mistaken, exceeds the width of the 135 gallon. An adult arowana may feel cramped in such a situation.

That's a big clown knife, but unless it's stunted it's going to get bigger; in fact, I'd say they get larger than arowana, especially since their body is much deeper than that of a silver arowana. My LFS has one thats pushing two feet, and they get even bigger than that. In a 180 gallon this clown knifefish isn't able to turn around easily. Unlike arowanas that are much more flexible, knifefish like to keep their bodies straight, which actually means that they need more room to swim. Eventually, the clown knifefish will need a larger aquarium. This fish is the one that really worries me, because otherwise you'd be able to put all kinds of fish in your tank without having to worry about this fish eventually being large enough to eat them.

I'd also leave out the ray. Plecs have been known to suck on the skin of rays, which is primarily the reason I don't have one already.

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The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.

Rays can be a bit tricky and teacup is not a good term to use. A teacup is technically a young ray and the name is used by stores on all sorts of species. Just telling someone to buy a teacup ray could get them just about anything and it probably isn't going to stay small.

The potential adult length of a silver arowana(Osteoglossum bicirrhosum) listed on most reliable sites is around 4'. It's not going to fit in your tank. They really should not be kept by any normal fish keeper since you can't buy a tank that's much more than half as wide as they are long. Most of the poor things probably end up quite stunted which would explain the variety of potential sizes listed on some sites. For them to be able to easily turn around and do more than float there you'd have to custom build something. Better start looking up plans for a bigger tank http://www.garf.org/news25p2.html#HUGE or flood the basement if you really want to keep fish like these.