Ruck don't sell fans to the hydro market under the 'Ruck' name any more, they re-branded as 'Can-Fan' for the hydro market.
So you need to buy a Can-Fan, it'll come in a Can-Fan box but the sticker on the actual fan will say "Manufactured by Ruck gmbh" in small writing, they're exactly the same units as they've always been they just come in a different box with a slightly different sticker on the unit.
the model letters after the numbers that designate size indicate if the fan compensates for the pressure drop when a carbon filter is fitted and also if it has a 4 step controller built in.
For example.......
A Ruck/Can-Fan 125 ............... is just a regular 125mm fan.
A Ruck/Can-Fan 125L ............. means it is a 125mm fan that is slightly higher powered (L) and has an internal circuit that will compensate for the pressure drop when a filter is fitted (stick a filter on the end and the fan auto spins up harder, it's quite cool to see).
A Ruck/Can-Fan 125LS ............. means it is a 125mm fan that is slightly higher powered designed to be used with a filter (L) but it also has a built in 4 speed step controller (S) fitted.
So you want to make sure that whatever size Ruck/Can-Fan you buy it has LS after the numbers.