When to upgrade skimmer?

[whatcaneyedo]

If everything else is optimal but your tank is still a slimy algae covered mess then it would be a good idea to upgrade the skimmer. But as Mitch suggests, adequate water flow throughout the display and the rest of the system is critical for getting waste to the skimmer in the first place. A friend of mine once had a very clean running 500gal and then downgraded to a 75gal that was always filthy. He used the same massive skimmer from the 500 but the water flow throughout the new system was so poor that very little of the waste ever made it to the skimmer to be removed.