I'm planning on getting a 20-29 gallon tank and I was wondering if I should get a sea cucumber or a sea urchin or both. I was also planning to get a ocellaris clownfish and a Jawfish/ crustaceans are they all compatible?
The sea cucumber is a great cleaning crew member for your tank by keeping the sand bed clean. But when it dies it will emit a toxin that will kill everything in your aquarium unless you get it out fast enough. The sea urchin is not a great choice because when it gets bigger it will knock over rock, corals, and may harm the other critters in your tank. The sea cucumber is probably the better choice but keep your eye out for it and make your tank conditions very consistent and at the right levels. But I would stick with snails and hermits for now as your cleaning crew until you get more experienced.
Most sand-sifting cucumbers are perfectly safe in the aquarium, whilst there is a risk, it is minimal. Here's some interesting reading about cucumbers
I've had a cucumber in my tank for months, it happily goes about its business, cleaning my sand bed. I don't think a cucumber needs any special experience, other than to plan your tank, or specifically your rock work. Be sure the rocks are 'buried' in the sand, so that if the cucumber decides to try and tunnel under the rocks, it can't, thus avoiding an avalanche. The same methods should be applied if you are stocking any sand sifters (gobies, nassa snails).
Urchins, I'm not really familiar with. Some are suitable for a reef tank, others can devastate a reef. Research is required into urchins, and what are safe. From memory, some also eat clams and can force a shell open.
Very interesting would have read it sooner but I have been busy good read. I'm not sure now what to decide I feel 50% both ways on the matter. Thanks for the link.