DIY floating sandy beach/lillypad for crabs

a DIY solution I came up with years ago was to take a thick nylon rope, say 2' long, knot it a few times on each end, put a suction cup or twist tie on the top end, and wrap a plant weight around the other end. I would then hang these from above the waterline on my fixture or tank brace, and simply anchor them with gravity to the tank substrate. using several of these, the crabs can climb up and get all the air and bask in the heat from the fixture. the benefit of this nylon rope trick is it minimizes the amount of light being blocked to your plants. you can find such rope at any major crafts or home hardware chain, or simply buy it by the foot from ebaY resellers - just be sure to get something braided and more than a centimeter wide or thinner and triple braid it yourself. thickness or braiding are needed for the crabs grab and climb - this is also a great way to isolate crabs from crayfish, as crayfish find it more difficult to climb then the more center gravity crabs.

note the crab on black nylon rope photos on the first page of this thread;
http://www.plantedtank.net/forums/diy/40170-moss-rope-ladder.html
ignore all the stuff about the moss - just focus on the rope structure itself.

Burr, i think it might be time to clean your filter media.......

Burr, it just occurred to me (can't edit my first post anymore) that you can combine a foam ball with a nylon rope in case you worry tying the rope to anything at your tank top may make the crabs able to escape your tank. there really is no need to cut a dish into the foam, since they will use anything they can get a grip on, so a foam ball, egg, or dome (half ball) they sell at every crafts store will do nicely (best choice would be half ball or half egg). a crab naturally grips when he's relaxed, and uses efforts to release or grip harder - so they can literally "sleep" gripping something while hanging upside down.

I have never observed any invertebrates eating moss, as some eat dead plant debris and others eat algae, but rarely any plant fresh and alive. again, don't get distracted by all the moss talk on my linked thread.
do a forum search on "foam background" or "3d background" for more on working with foam in a planted tank. personally, I have never worked with foam since in my case the crabs could not climb out as I stuck my rope to the bottom clear cover of my tank fixture. when I need some gimmick to make a tank item float, I generally use fishing floats - but they may not help as you want a pitted surface for the crabs to climb on.
you can also try cork - it's as effective as foam at floating, is a wonderful climbing surface, is natural and won't fowl your water. try a crafts store for cork, or ebaY - just make sure it's not sealed or treaded with any chemicals - the more untreated the better. over time cork will degrade, water log and dissolve, but that could be years from now. when working with cork as a flotation device, always keep at least half the cork mass above water - don't immerse the whole thing or it may eventually sink.
dry Cyprus wood also floats, is fish tank safe, and will take years to degrade. it's often used as a floating key-chain at the dockside. again, keep half of it above water for the same reasons.
personally, I would not use foam at all - and try to pursue a more natural floating materials option.

you could paint the styrofoam with krylon fusion paint, which is safe for in aquarium use once fully dried and cured. i'd leave items painted with krylon to dry/cure a good while before putting them in the tank. i'd do a week (but i'm a paranoid fish mommy, lol).