how many harlequins do you have? they get larger than you think. I have one that's over 1" long, the others havn't caught up yet but they're not even 1 year old. If you had 3 harlequins you could add maybe 3 other small tetras, never heard of candy or diamond gold. If the"diamond golds" are diamond tetras, they get fairly large and I'd avoid them in a 10. Diamond tetras are white with a shimmery body that looks like you sprinkled diamond dust on them or something. Something small like neon tetras, cardinal tetras or even glowlight tetras would be ok. I don't think you'd need a large school of the small tetras like usually recommended. The harlequins are quite gutsy and will swim about no matter what's going on. they should "break the tension" for the smaller tetras. Kyle
oh, so you don't have any yet- I don't know, I like mine a lot. They look great in a planted tank. Do you mean the gold tetras which are like black neon sized? I don't have any personal experience with them, but from what I understand they are pretty hardy. I think as long as you chose from the smaller types of tetras - the neons, the glowlights, rather than the serpae type tetras you will do fine. It's mostly a matter of your preference there- the little tetras are all peaceful fish comparable to harlequins.