You have just verified the need to buy the right chain for your bike. It is determined by the number of gears on the cassette because as the number of gears increases, the width of the tooth is smaller. If you look up chains on eBay, you will see that sellers list 6-8 speeds as one chain and 9 thru 12 each a different chain. Here is what Wikipedia shows for chain width:
- 6 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
- 7 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
- 8 speed – 7.3 mm (9⁄32 in) (Shimano HG), 7.1 mm (9⁄32 in) (SRAM, Shimano IG)
- 9 speed – 6.5 to 7.0 mm (1⁄4 to 9⁄32 in) (all brands)
- 10 speed – 6.0 to 7.0 mm (1⁄4 to 9⁄32 in) (Shimano, Campagnolo)
You will see that 6-8 are the same but all the rest are different. Buy a 9 speed chain. Sellers also list the number of links in their ad. Here is one for $16 postpaid and it is a decent chain, one that I use in the 10 speed version on my recumbent trike.
https://www.ebay.com/itm/304660205330