Well, a "nano" tank is anything from about 10 gallons to 30 gallons... So, cost and available livestock are going to vary greatly... My 10 gallon is probably nearing $1000, counting all of the fish and coral that I've added in. I stopped adding a long time ago, but I know that last week alone, I added $50 worth of new coral (that got me 3 pieces).
The initial setup isn't all that much but if you want anything *in* the tank, that's where the cost is gonna get you.
In my 10g, I have 3 fish (but 2 is the recommended max - long story on how I got 3) and have several soft corals and snails and hermit crabs. If you go up to a 30gal, you can add more fish, but you're going to at least triple your setup costs b/c a large chunk of setting up a tank is in the liverock. It's recommended you get at least a pound per gallon of LR and at an average of $7 a lb, that can get pretty pricey.
Great tank, also has the tank name which you can search for on Ebay and buy for around £40, He also says the overall cost of his project. As its relevant to England you can inclue similar equipment to him.
well I spent close to 400 bucks setting up my 10 gal nano. when i upgrade my lighting to get corals itll be about 100 bucks ill also need to get a skimmer that hangs on the back of my tank rather than in the tank.....if there is such a thing. check out my diary along with parker313's and SH's
I don't know anything about keeping seahorses, but I know that they can be very delicate and tough to keep alive. I know I wouldn't be up for the challenge....
Again, the cost depends on what size tank and what equipment you choose - there's really not even a good way to ballpark it without knowing what size you're talking about.
Lighting - again, depends on what size tank you have and what equipment you have on hand already. I had a hood that fit my tank and just had to get a new bulb. The lighting can be pretty expensive though, especially if you want to keep some of the hard corals.
Filtration - live rock is the main source of filtration in a nano, that's why it's important not to scrimp at all there.
Have you read the topics pinned to the top of the Marine & Reef Chit Chat forum? They aren't just for large tanks, there's a lot of good information for anyone thinking about saltwater at all.
Look up captive bread seahorses, I plan on a pair for my 29gal. Right now it’s a FW tank but soon to be a SW. Captive bread (CB) seahorses are the best they are way easer to care for then SW. I would recommend one Pair for seahorses for 29 gal but it seems that a lot of people can keep a pair per every 10gal. And keep them very healthy and breading. So I guess you could keep 6 seahorses in a 30gal tank but I would not recommend that at all. Go to seahorse.com