Saw that I had ext3 running on some of my SSDs which supposedly does Not support TRIM. Time to convert them to ext4!
fsck.ext3 -pf /dev/sdb1
tune2fs -O extents,uninit_bg,dir_index /dev/sdb1
fsck.ext4 -yfD /dev/sdb1
Then change the mount options in /etc/fstab:
/dev/sdb1 /mnt/sdb ext4 defaults,noatime,discard,nodiratime,errors=remount-ro 0 1
All done! Now let the TRIM-ing begin! :)