#ed_op#DIV#ed_cl#Hello,#ed_op#/DIV#ed_cl##ed_op#DIV#ed_cl##ed_op#/DIV#ed_cl##ed_op#DIV#ed_cl#welcome to the club sounds like you have a good start...For a fish only (FO) tank you can use just about any substrate you want, however different substrates have different uses. In general for a marine tank its best if you use argonite sand.It will help to keep th ph and calceum levlesin check and help with filtration. a 2-3 inch deepsand bed should do the trick. If im not mistaken you said you have a 4 foot 20 gallon wall tank? being so long I cant imagine its very wide or tall for that mater so the use of live rock might be difficult but if you can squeze some in do it. The primary waste product of marine animals is amonia the live rock will help with the removal of this. Also your canister filter may have activated carbon in itwhich will also help with removing amonia and other stuff. The last thing I should tell you is that with only 20 gallons overpopulation can be a big problem so you should keep only about 6-8 small fish. Some samller species of fish include golbies, blennies, some kinds of demsles (careful they tend to be very teritorial), and some kinds of clowns (including nemo aka ocelerous (sp)clown). In addition there are many helpful articles posted on this web site that will help. I recentaly posted one under the heading "The Berlin style of reef keeping and its modern applications" that may be of some use. With that I wish you luck.#ed_op#/DIV#ed_cl##ed_op#DIV#ed_cl##ed_op#/DIV#ed_cl##ed_op#DIV#ed_cl#Tigersax7#ed_op#/DIV#ed_cl#