FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article talks about Forward Error Correction (FEC) feature under SFP slots on the FortiGate-180xF.

For FortiGate-180xF (1800/1801).

There is a possibility to define different speed/duplex setting on the interfaces 25 to 36 on FortiGate-180x.
These interfaces support 1G, 10G and 25G speed.
FortiGate-1800F (port25) # set speed
1000full     1000M full-duplex.
10000full    10G full-duplex.
25000full    25G full-duplex.
In order to use these ports, forward-error-correction has to be disabled as follows.
FortiGate-1800F (port25) # show
# config system interface

    edit "port25"
        set vdom "root"
        set type physical
        set snmp-index 25
        set forward-error-correction disable <----- Not supported by FortiManager until version 6.2.8, 6.4.6, 7.0.0.
        set speed 10000full

FortiGate-1800F # diagnose hardware deviceinfo nic port25
Description     :FortiASIC NP7 Adapter
Driver Name     :FortiASIC Unified NPU Driver
pid             :25
oid             :88
vid             :26
macid           :24
eif_id          :127
promiscous      :0
mtu             :1500
netdev oid      :88
dev-flags       :1003
dev-promis      :0
Current_HWaddr   04:d5:90:d5:4c:f0
Permanent_HWaddr 04:d5:90:d5:4c:f0
==== Default Link Settings =====
auto-nego       :Disable
s_speed         :10000
s_duplex        :Full
==== Current Link Settings =====
auto-nego       :Disable
s_status        :Up
s_speed         :10000
s_duplex        :Full
FEC             :OFF (0x4)
FEC_cap         :CL91  (0x8)
==== Link Status ===============
Admin           :Up
link_status     :Up
Speed           :10000
Duplex          :Full
==== Netdev Status =============
dev_running     :Yes
dev_carrier     :On
==== Host Counters =============
rx_pkts         :6
rx_bytes        :648
tx_pkts         :2
tx_bytes        :180
tx_drop         :0
tx_e_busy       :0
tx_e_noheadroom :0
tx_e_oid        :0
tx_e_adapter    :0
tx_e_other      :0
==== Switch Poll Counters ======
tx_pkts         :1
tx_bytes        :94
rx_pkts         :0
rx_bytes        :0
==== Netdev Counters ===========
Rx Pkts         :0
Rx Bytes        :0
Tx Pkts         :1
Tx Bytes        :94
Related link.

Related to FortiManager known issue 691240 – 'FortiManager should not unset the value forward-error-correction with certain FortiGate platforms.