Hey everyone,
Hello 
, welcome to MFK!
I am planning on starting a 10 gallon FO tank. Need some guidance and advice.
Cool cool. Well you came to the right place!
It is currently african cichlid FW
Hmm pretty small for those guys, babies I assume?
and I am using a 20g aquaclear filter on it with crushed coral as added media.
Mmmk well if you intend on using that same filter for biological filtration, you need to dump the media and clean the chamber and intake really well first. And toss out the crushed coral, it will mess with your parameters and attracts diatom algae.
I have a 75w heater on it and a marine white sand, not live.
K, well the heater is plenty. When it comes to the sand, it's also fine, but just keep in mind that when using bare sand (as opposed to live sand), you are extending your cycling process and will have to introduce nutrifying bacteria by hand to aid in the cycling and colonization of beneficial biological colonies. Live sand contains a plethora of these guys and can efficiently provide for base populations to take hold and colonize, but bare sand does not and therefore you will have to add it. A good brand for this is CYCLE, and it's easy to dose and pretty much impossible to over-dose.
All I want is a single clown fish in there.
That's a pretty small space for a clown fish, but if you stick to the smaller species such as False Perculas which are Ocellaris Clowns, you will have more luck. Also note that you will need some form of protection for this guy, a place to rest and take up shelter. Exposed fish are prone to stress, if you do not wish to have any live rock, consider at the least, a host anemone for this guy.
I am planning on using RO water from the water vending machines outside grocery stores and I plan on buying instant ocean salt is it ok so far?
That RO water you mention is fine but make sure that you allow it time to aerrate before doing water changes later on down the line. A lot of times, even though the water its self was thouroughly filtered through reverse osmosis, chemicals such as chlorine, and now, more popular chloromine is used to limit bacterial growth in the standing water. These dissapate but need to be exposed to air and circulated for several hours first.
what else do I need to put inside the tank?
Well since you are opting out of Live Rock - which is a natural filtering aid, and since you are using bare sand instead of Live Sand, you will want to think about purchasing a protein skimmer to help remove biological waste and solid particles within the water. Your biological filter alone cannot do this.You will also need a source of current/circulation. The return flow of the vast majority of filters is far below sufficient to hold this job, so a decent power head should be added into the water. Not only does this keep your water well oxygenated, but it also keeps the filtered water moving and the "bad" water being brought into contact with the filter intakes, and keeps detritus (waste) from settling on the substrate bed.
Can I put more than one clown fish in there once it has cycled?
No, sorry, I would not advise that. However, if you change your mind and instead of a Clown, are interested in other residents which do well in such small quarters, let us know.
when it comes to cycling do I just leave it the way it is for how many weeks or months?
Well like I mentioned previously, you are already going to be extending your cycling period since you are not using live rock or live sand, so you should expect a wait of between 7-9 weeks until the tank is fully cycled and has had the opportunity to establish bacterial colonies within the filtration media. Are you familiar with the Nitrogen Cycle? I would assume so since you said you have kept freshwater. Well with saltwater you will need to make sure that you are testing your parameters on a regular basis so that you can track the progression of this cycle. Once you are steadily reading 0-Ammonia, 0-Nitrites, 0-Nitrates, and have obtained a steady PH of 8.2+ for a period of at least 2 weeks, you will know your cycle is complete and ready for the addition of tankmates.
when would it be safe for damsels to actually start it up for while? (I do not want to introduce them to their doom on the first week, but rather to have something in there when it is safe for really hardy fishes)
There are a few other ways of finishing a cycle, that do not include the use of fish, but if that is the way you prefer to go, I would say several weeks after the set-up of your tank, and after the ammonia spike has taken place. This goes hand-in-hand with regular testing of your parameters throughout the cycle. If you were using live sand, and live rock, it would be sooner, as both of these (as previously mentioned) speed up the natural cycle. If that were the case, you could have live sand, and at about 3+ weeks start adding live rock, and at about 5 weeks add the damsels. But since you are not, I would advise against it until you are able to see the progression in the cycle through testing and pass the point where ammonia peaks.
I already talked to my lfs about getting some store credit to use for the clown fish when I trade the damsels back in.
That's cool, and at least you are thinking ahead. Damsels can be notoriously territorial, especially in a space as small as a 10gallon, and would not be too friendly to the Clown.
I have been doing FW for a long time, and I have been reading about SW for a while. I just thought that the only way to learn more about it is to actually start one up.
Very true, and it's nice to know that you have been doing your research and are taking an educated leap into this!
Pardon the long essay.
LOL hunny you haven't seen long yet. 
I appreciate any input!
No problem at all, hope this helps some. If you have more questions, please, ask away!
