Is a sump an absolutely necessary thing to have? Im starting a new SW tank in my old 30 gallon tank and am wondering if i really HAVE to have a sump. thanks.
You have to be careful because a sump/fuge increases the water volume which means you can have more fish even though they are kept from the sump/fuge. What this means is a display could have a bunch of fish in it looking like it's full of life. If you try these same densities in a a non sump/fuge system you will be treading on thin, fragile ice.
So no, you don't need a sump/fuge. You will have to have fewer inhabitants than you could otherwise. If you don't need the packed-full-of-life display you won't have a fragile situation without a sump.