i dont think the hydor inline heater will work on a fx5 as they have the big ugly ribbed hose
the hydor 300w inline take 16-22mm hose
no matter what heater you have you need a gaurd on it just because your ray hasnt been burnt yet doesnt mean it wont get burnt why take the risk heater gaurds cost $5 rays cost hundreds
the tronic heater gaurd will fit on any heater
i have been running 2 x hydor 300w one on each of my eheim pro 3s for 2yrs no problems they warm the water faster
I have looked and looked for tronic heater guard, everyone online is out of stock! Found one site, but at checkout my computer alerted me that they were a hacker risk.