Originally Posted by rustystrings61

Your hub is a double-fixed flip/flop hub, but that's not an issue really. I've run a freewheel on my double-fixed hubs like that with no ill effects. Looking at your bike I think that if you add a 20T freewheel to the other side of the hub and then run that instead of the 13T fixed cog you will no longer need the tensioner. I would then remove it, because it's just added weight. Your chain length as it is should be fine - it's too long as it is for the 13T, and someone was lazy and used a tensioner rather than set the length where it needed to be.

Tensioners are nice for bikes with vertical dropouts where you can't move the wheel back and forth within the dropout to adjust chain tension. I ran one on an old mountain bike I converted to single speed, but that was with a freewheel setup. They're actually NOT a good idea on a fixed-gear as the chain is under tension in both directions.

Anyway, I would grease the threads on the hub and thread on a single-speed freewheel and go for a ride and enjoy it. 50 x 20 with 25 mm tires would yield around 66 gear inches, which is pretty close to what standard practice used to be for single freewheel gearing. And if you feel adventurous, I would take the bike to a good shop and swap out the 13T cog for a 19T unit, just in case you want to try fixed-gear riding with a more useful gear. 50 x 19 would get you to around 69-70 gear inches, which as been my preferred fixed-gear setup for 20-odd years now riding the rolling terrain in this part of South Carolina.

Just because I don't like it, I would consider replacing the chain with a conventional one, but that's just me. But if it works okay, then just run what you have until you wear it out naturally and then decide what you want to replace it with.