What's the original sealant? I guess this depends on how old the tank really is.
Vintage metal frame tanks often used black tar as sealant. You can certainly remove it and replace it with silicone - use black to get the same look. If it is tar, you will need to make sure you get every little bit of it so the silicone will adhere.
I think the metal frame tanks have a certain elegance to them. I'd choose a fish that was similar, possibly long finned, like threadfin rainbows.