Depends on the type of clown and/or goby. But, my tank was looking pretty stable after 3 months so I tried a small crocea (3-5 inch). did fine, still looks great 4mos after that. Now I've got a small derasa (2-3in), a small squamosa (2-3 in) and a bigger Ultra Crocea(5-6in). All doing great. No fish bother them, except my jawfish who always seems to tunnel under the large clam.
Also, be sure you have enough light.