Which ink does the Epson EcoTank ET-16500 use?

The Epson EcoTank ET-16500 (ecotank, 2016) uses Epson 664.

Compatible cartridges

CartridgeColorYieldPagesOEM priceCost/page
Epson 664 black standard4000 $13.00 0.3¢ OEM · AmazonCompatible
Epson 664 cyan standard6500 $13.00 0.2¢ OEM · AmazonCompatible
Epson 664 magenta standard6500 $13.00 0.2¢ OEM · AmazonCompatible
Epson 664 yellow standard6500 $13.00 0.2¢ OEM · AmazonCompatible

XL/high-yield cartridges usually cost more upfront but lower the cost-per-page — better if you print often.