I know this thread is super old, but wanted to share some information I was able to find. I just purchased a new RSM 250c that I found in leftover inventory and got a great deal on it. I loved this tank years ago when I had the gen1 version and enjoy the size and it being closed top. I remembered the fans for the bulbs being extremely loud, so read this thread and others to find information about the fans and installing better, quieter fans. I ended up finding the specs on the stock Red Sea fans and I'll share the specs on the fans I installed below.
Red Sea Stock 60mm x 20mm: 9.9cfm | 3000 rpms | 20db
Silenx iXtrema Pro 60mm x 25mm: 18cfm | 2000 rpms | 16db
The Silenx fans fit without modification on the C series tank, because the fan holders are open, not rounded and closed off like the original fans. The specs show these fans push more air, however, it doesn't feel as though they are pushing more air, I believe this is because the SilenX are better designed and you aren't going to feel it as much, although it's moving more, and I believe it's because the stock fans move at 3000 rpms, which gives the feeling it's moving more, when in fact it isn't. The difference in noise is amazing, now I'll be curious how they do while running once the tank is up and going. Just thought I would share for anyone who may still be using the RSM 250 and be looking for more current information.
