An activist short-seller slammed Planet Fitness and the stock is tanking

Activist short seller Ben Axler, the founder of long/short equity hedge fund Spruce Point Capital Management, is short Planet Fitness.

The fitness club chain's stock fell more than 10% in the after-hours session and was last trading around $13.20 per share.

Speaking at the Harbor Investment Conference, Axler said he thinks the company has a "questionable business strategy."

He said the company has "unachievable revenue goals," saying the company wants to run 4,000 clubs with a membership base of 6,500 per club.

He also called the company a "glorified equipment reseller." He said that the company has unsustainable margins, no barriers to entry (a lot of "copy cats"), and rapidly rising competition in the fitness space.

On the accounting side, Axler said he thinks that the company has some issues with its equipment purchases/resales.

Axler has an $8 price target for the company's stock.