I'm planning to set up a "display" type of tank. Any recommendation as to the size of substrate that i should use? i plan on using round smooth type of material, i.e. pea gravel or roundish pebbles.

i plan to set it up for Orandas.

Welcome to Kokos!

Many people - including me - use very little substrate. I use it ONLY for decoration. I use what is called "river rock". These actually are hand picked stones of larger size that are scattered or placed around the tank stategically for effect. They allow easy cleaning and the fish can pick around them well.

Gravel of any size that is from twice the size of a fish's mouth or smaller can potentially be picked up by a goldfish and stuck. It is, unfortunately, a common problem when a goldfish swallows or gets a stone stuck in their mouth. This can be a serious problem if it is not addressed quickly.

Orandas are one of the gentle giants of the goldfish world. They can easily get to 9-10 inches long with mouths that can swallow a stone 1 inch plus in diameter. I would aim for stones of 2 inches or greater.....

If you have too many of these, though, you will not be able to clean through them with a standard vacumn. Stones of this size are better used sparingly....