Install the following browser extensions:
Keyword Surfer (if you wanna see search volume)
Search "[your niche] forums".
Ignore any results that aren't actual forums.
Hover over the metrics provided by Keywords Everywhere (first step in the screenshot) and click either number on the right of the panel (they both lead to the same page).
That'll bring up a list of the keywords bringing that forum the most amount of traffic. If UGC (user generated content) ranks well, there's a good chance you can as well, even with a low authority site - just as long as your content is thorough and matches user intent very well.
If the forum is large (as is the case with the example above), the first bunch of keywords are still gonna be very competitive.
So scroll down and go through the next bunch of pages. You're bound to find some long-tail, obscure seeming keywords with little competition.
I only looked through a few, but on page 11 of FishLore's keywords I found "signs your pleco is dying", searched it, and there are multiple sites ranking on page one with a DA of less than 10. Good sign the keyword isn't very competitive.
Also, 210 estimated monthly search volume if you care about that :)
Let's try with a smaller forum. We wanna find really obscure keywords :)
This time, I'm looking at fishforum.net’s keywords.
On the first page, there's several decent looking keywords already.
"angelfish with red eyes" (90 SV, low DA sites, lots of UGC)
"purple algae in fish tank" (110 SV, low DA, UGC)
And on page 4, I found "aqua tech 5-15 filter not working". This one's SOOO good.
Relevant results get ok traffic according to Keywords Everywhere. Low DA sites ranking. UGC abounds. A PINTEREST PIN??
Easy, easy, easy.
You can take this method even further, but that's enough for now. Perhaps another day :)
Want more cool keyword research/SEO tips? Follow me on Twitter @IanNicheSites and consider taking my course, at smartkeywordresearch.com!