I recently bought a used tank and filter from Craigslist and even though the seller didn't mention it I have a sneaking suspicion it was used for a saltwater setup. What's the best way to clean the HOB filter so it can be used for tropicals?
I'd just go with a TON of rinsing. Don't expect any beneficial bacteria to live or be the right freshwater kinds. I also do a bleach 1:10 wipe/soak, then rinse just to be sure of killing any pathogens... & again a ton of rinsing & Prime tx x 2 or 3.