I know you need to forward the ports you'll be using for bittorrent, but after a reinstall of windows I never got around to it because I seemingly didn't need to, my torrents were downloading to the limits of my bandwith, but then I noticed this issue that sometimes they'll stop seeding even though there are still leechers in the swarm.

I had this problem on utorrent (and some other problems) and so I switched to Tixati, but this still happens.

It then occurred to me that it may be due to port forwarding so I looked at that Tixati optimization guide and it said "put a torrent to download and if after 5 minutes you see '0 incoming connections' in the lower right hand corner it means you're not connectable, if you don't then you're good" so I did this and there wasn't a "0 incoming connections" message (but it didn't say how many incoming connections there were, either, but I guess that's normal, no?).

However, I then go and look at my ports (and it says in Tixati "UPNP forwarded" next to them) and I go to three different sites for checking and all three say the ports are not open. Then I go and check on all three the port I was using for incoming connections on utorrent, the same.

Now I don't get this, how was I getting downloads at full capacity, and, when the uploads were working, uploads at full capacity if none of the ports the clients were using were open?

Do I need to open the ports or not? Could my problem of clients randomly deciding to not upload, even though there are leechers in the swarm, be due to port not open?

No, you don't have to forward port to be able to download, however with port closed you won't connect to some peers at all. Often you won't even notice any difference, there will be enough connectable peers to saturate your bandwidth. Sometimes you may not be able to complete a download even if it is seeded. It's just like described in Optimize Tixati:

"In many swarms, it is typical for nearly half of the peers to not be able to receive incoming connections. This means if you are one of these peers, you are missing out on downloading from many other peers with your same situation. If you could receive incoming connections, these peers could connect to you and provide you with a faster transfer."

UPNP doesn't work with my router, I had to configure port forwarding manually. It may be hard only for the first time, portforward.com is helpful, there are a lot off free help guides there.

I see.

I found my router on portforward.com and it says I have to set up a static IP for my computer, but I'd like to forward this port for all the computers that connect to this router, because other computers use bittorrent as well. Can't I do that somehow?

I see that other guides also say I should set up a static IP, bu I think I didn't set up a static IP the last time I dealt with port forwarding, but that was a long time ago.

You setup static IPs on your router not your ISP. Configure each computer on the network to always use the same IP. On my router it is under LAN setup but I can also lock wireless IPs there. Find where to check the box to make the router a DHCP server. As said on mine it is in LAN setup. Each computer must use it's own port with no other computer sharing that port or the router will not know where to send the data too.

I don't know if this will help but these are the instructions from my router.
Use Router As DHCP Server

The router is set up by default as a DHCP (Dynamic Host Configuration Protocol) server, which provides the TCP/IP configuration for all the computers that are connected to the router.

Unless told to change these settings by your ISP, leave the Use Router As DHCP Server check box selected.

If your ISP has you clear this check box, you must have another DHCP server within your network, or else you must manually configure the computer.

   Starting IP Address. This field specifies the first of the contiguous addresses in the IP address pool. 192.168.1.2 is the default start address.
   Ending IP Address. This field specifies the last of the contiguous addresses in the IP address pool. 192.168.1.254 is the default ending address.

Address Reservation

When you specify a reserved IP address for a PC on the LAN, that PC will always receive the same IP address each time it accesses the DHCP server. Reserved IP addresses should be assigned to servers that require permanent IP settings.

To reserve an IP address:

   Click the Add button.
   Select the radio button of the computer you wish to add from the Address Reservation Table.
   If the computer is not on the Address Reservation Table, enter the IP address, MAC address, and device name of the computer you wish to add.
   Click the Add button when finished.


To edit a reserved IP address:

   Select the radio button next to the reserved address you want to edit.
   Click the Edit button.
   Edit the IP address, MAC address, or device name.
   Click the Accept button when finished.

To delete a reserved IP address:

   Select the radio button next to the reserved address you want to delete.
   Click the Delete button.

To save or cancel changes:

Click Apply to save the new settings to the router.
Click Cancel to discard any unsaved changes.

Sailor24 is right, you should set static IP addresses for each computer inside your LAN and different incoming port on each bittorrent client. Then configure forwarding rules. I would do it one by one, try to figure out how to do it on one PC first.

Thanks for the help guys!

Well, I set up a static IP on my primary computer and successfully forwarded a port.

I'm wondering now, given that I've decided I'll stop using bittorrent on the other computers so I don't need to forward separate ports for them, do I need to set up static IPs for them? Because some guides I've looked at said I should do this so the router doesn't accidentally assign the IP I reserved for my computer as its static IP to one of the other computers on the network, while other guides say this isn't necessary and the router knows not to give out the same IP twice.

Depends on your router, some do it automatically, sort of. Anyway I would do it, in fact I have done exactly that. My phone and  alarm clock even have reserved IPs and they hardly need them.
There are some advantages. No confusion, never see that dam error message that another computer on the network has the same IP so it is logging off(happens on restarts). I also now have it setup so if another IP is to be issued other than the ones reserved there is a confirmation dialog. This is better security and simple.

It is more secure because if you forward your ports correctly you maybe able to turn UPnP which is a security risk.
Better traffic flow for gaming if you are so inclined.

Alternatively you can set your PC's IP address outside the DHCP (dynamically assigned) range, this way it won't be in conflict with another device.

You got me. Sounds interesting though. How do we do that? Why are we doing that? Are we now outside the normal control of windows? Sounds like a hack, please be a hack!