I tend to avoid commenting on these threads but an aquarium of 18" wide won't be suitable for too long. Rhombeus are super slow growing but you are doing the right thing by passing on such a large species with the size restriction you have for the aquarium. As disappointing as this is (we've all been there) have a rethink on what you want that is more realistic. Everyone has their own ideas when it comes to what size is required for everything I bet if you looked through some similar threads you wouldn't have to look far before someone argues after not being given the info they want. Is it definitely a solo Serrasalmus you want? Pretty much everyone I've known myself included get bored eventually after 2-5 years of your Rhom not looking like the impressive pictures in the altas or online in more recent times and it just lurks 90% of the time.