Well I was hoping one of our saltwater moderators would have popped in to answer this but they haven't so I guess they've been to busy with the festive season.
No doubt as soon as I post this they'll pop in to tell me that I'm wrong )
Ok I would say that, its not possible to do what you want. I mean checking water parameters more than once a day? That would become kind of expensive. With saltwater the bigger the tank the better and you'd be best of with at least a 20 gallon I would say. I was always told that a skimmer was almost essential but then again you do hear of "nano reefers" who are able to support fish in the smallest of tanks. Like I said I'd wait for the marine mods to check this over before you do anything cause I may be way wrong I just didn't want your question to go unanswered for too long and to the best of my knowledge thats the right answer.
It's not very hard to keep clowns......5 gal is very small but i've see clowns in a 3.5gallon tank before, if you check you water that often then it should be ok just don't over stock the tank with other fish.Are they the only fish you are going to keep?
In answer to your question the answer is yes it is possible, while i personally wouldnt reccomend it, As william said the bigger the water volume the better marine fish need good quality and stable water conditions any changes to water quality would rapidly effect such a small voulme of water,
However this is how i would set up a tank that small, there is no point using a dsb (deep sand bed) as this would take a lot of space, i wouldnt use an under gravel fiter as these arent as effective as once thought i would use an external filter, which i would clean regularily, as they can become nitrate factorys if left to long, i would have a few pieces of live rock in there to aid filtration,
Finally the two clowns would be fine in there, i wouldnt add any other fish i would suggest maybe a cleaner shrimp and a few turbo snails for added interest and the also aid keeping the tank clean.
While water testing every day isnt necesarry i would perform water changes every few days, also i have seen plans for a skimmer suitable for a small tank if i find the link i will post it here.
Of course there is always the possibility of adding a sump to the tank which could increase the volume of water in the system
OK IMO, nano reefers,(sub 20gal tanks) do it for the challenge. I would get a hang on back filter with an integrated skimmer. These can be had in the states for under 50 bucks. Not sure where you are located though. Clowns also go hand in glove with anenomes. You will need strong lighting for these fellas. With a five gallon, you are pretty much limited to a power compact lighting system. Any invertibrates require optimum water conditions, and good lighting to flourish.
Not required by law..lol But always be prepared for the next step. With a five gallon tank, you will progrees through the stages of keeping marines quicker than the peeps with larger tanks. Before you know it, you will be thinking of what you can get next...