Jack’s Magic Stain ID Kit for about $15 is the only one I'm familiar with. But if you do in-fact confirm it's copper, you may have heard the metal routine before but I'll toss a few reminders your way:
- Removing/exchanging the metal-filled water is always idea (if possible) to avoid continued sequestrant use later; or at least maximum water change to minimize the effects
- Probably best to allow pH and FC to ride a little lower which might help reduce the precipitating effects between chlorine and metal; no doubt your last FC increase prompted this reaction. Hopefully you are algae-free, otherwise you'll be starting all over. So definitely make sure your water is algae-free before trying to manipulate the metal in the water.
- If you keep the water (or most of it) and continue with chemical management, ProTeam's Metal Magic and Jack's Magic the Pink Stuff (regular) and the Blue Stuff (fresh plaster) are some of the top sequestrants. You're right, they are not cheap and will continue as long as you have that metal-filled water (if confirmed). You can also find other brands with similar products, some of which are noticeably less expensive. Sequestrants based on HEDP, phosphonic acid, or phosphonic acid derivatives are the most effective.