I have been keeping freshwater fish for quite some time now, and I would really like to try saltwater. I really do not have room for a very large tank (like 75+ gallons). What would be your recommended size?
That's going to be a hard one for people to answer. We don't know what size you have space to keep, nor what size you're *inclined* to keep. Saltwater, IMO, is more of an investment in time, cost, and effort (assuming you don't have your own RODI setup, for example) than freshwater, but the fish are generally much prettier.
The same rule of thumb for tank sizing applies to both fresh and salt: the larger the tank, the more room for error. When a small tank starts having water quality issues, it goes downhill fast. My 10 gallon betta tank had a shrimp death, and in hours (2 shrimp at bed time, 1 and a shrimp tail right before lunch), ammonia was at 1.0 or so. Fortunately, I was home, and noticed odd behavior from some tetras that are in with him. On my 55, that would have been a much smaller spike, just due to sheer water volume. And that volume packs much more beneficial bacteria, so it may have evened out before I noticed.
The better question is what do *you* think you're capable of and willing to handle? Then, some advice can be given. Unfortunately, I'm not really a saltwater guy, I just admire them and have learned a few things through observation. I can even answer some hardware questions, but stocking, especially corals, I won't go near. I just lack the familiarity.
Looking at some recent activity, it looks like the people I'd be inclined to lean on for information haven't been on in a few days. Be patient, and they'll come by and will happily advise.
Personally, I'd go in decreasing preference for tank size (again, pending on what you're wanting to deal with and only referencing fairly standard sizing) 75>40 breeder, 29, 55, 20 long, 10. Bowfronts are nice options, and there are some biocubes that are fairly non-standard in size, but may fit your needs. I think some of those even come with built in sumps, which not only filter, but being that they also hold water, they help give you a larger body of water, so parameters won't swing as wildly.
I am looking to get a tank that is not so small it has really high chances to cause problems, but not so large that it costs a fortune to maintain. I was thinking around 50 gallons would be good. I already have a 50 fresh, so size wise, I know what to expect.
Now comes the hard part of deciding what route to take, Fish Only, Fish Only w/Live Rock, or Reef. I'm not sure if there are other options, but that's what my brain says. And you need to decide on whether or not to go with a sump. All kinds of fun stuff.
And then will be the harder part of what to stock with.
Oh, yeah. Covering a saltwater tank is more of an issue, because evaporation can cause your salinity levels to climb (in case you didn't know, the salt won't evaporate) due to loss of water, and that can be harmful to your fish.
One of the things I strongly recommend is that someone who is looking at Saltwater spend some time checking around www.liveaquaria.com They are pretty accurate for minimum tank sizes required for fish, whether or not a fish is reef safe (will eat corals and/or inverts) and the general care level required for a given fish. This can help you narrow down what size of tank you want. I would agree that generally bigger tanks are more stable, however if you go really big, the sheer cost, maintenance on the the tank and equipment, etc. can get overwhelming really fast. Opinions vary but IMHO 55 gallons is about the tipping point between HOB and Sump (if it's under you can go HOB and if it's over then it makes more sense to go with a sump). 20 Long is usually the bare minimum I recommend for people starting their first saltwater tank as it makes the best use of space for the size, 40 breeder would be the next in line and then I usually recommend going up to 75 gallons. The reason I skip the 50-55 gallon tanks is that they tend to be very skinny, which doesn't offer a significant increase in stocking options and as mentioned is usually right around the tipping point between HOB and Sump. You may notice I didn't mention canister filters. Why? Because Canister filters are the most labor intensive and problematic means of filtration for saltwater. They are very prone to trapping the nasties and becoming out of control nitrate and phosphate factories. If you go HOB, the Fluval (formerly Aquaclear) HOB filters are highly recommended due to their wide range of media that they can handle (ie Chemipure Elite) and the fact that they can be easily modified into a macro algae refugium (ie Chaeto growth) to filter nitrates and phosphates for you. For sump, I recommend no less than 20 gallons since smaller sumps simply don't offer the additional water flow, space for solid protein skimmer and return pump, etc. to make it worth it. Small sumps are also a big time pain to clean and remove equipment from (which you will need to do periodically). I started with a sumpless 36 gallon saltwater tank with a Fluval 50 HOB filter and an Eshoppes PSK-75H HOB Protein Skimmer and then moved to my current 90 gallon pre-drilled tank with a 29 gallon sump. That is the other thing, if the tank isn't pre-drilled it does add a complexity factor to adding a sump as overflow boxes tend to have siphons that clog and it can often be more hassle than it's worth.