A very WARM WELCOME to MASA turboguppy! I am very glad that you found us, before actually starting out your marine tank.
Look - yes - it is possible to convert your current tank, but it might be worth the hassle.
Here's a short list of things you should read up on, which could lead you to asking questions and learning, BEFORE you buy a tank, or set out to modify your current tank:
1) filtration: sump, skimmer, live rock, chemical filtration, biological filtration, mechanical filtration, DSB (deep sand bed), algae scrubber
2) water parameters: nutrients, phosphates, nitrates, denitrification cycle, aerobic and an-aerobic bacteria, salinity, pH, alkalinity, calcium, kalkwasser (calcium hydroxide)
3) lighting: T5 high output fluorescents, metal halides, lighting requirements of live stock, actinics, Kelvin rating, natural daylight
4) water flow: power-heads, dispersed water flow, detritus settlement, water volume turn-over
5) plumbing: down-pipe to sump, Durso stand-pipe, closed loop system, piping to sump/skimmer, piping FROM sump to tank
6) test-kits: pH, salinity/SG (hydrometer or refractometre), alkalinity, digital thermometer, calcium, ammonia, nitrite, nitrate, phosphate
7) substrate: both for in-tank as well as in-sump (SSB - shallow sand bed, DSB = deep sand bed), substrate/sand particle size (size matters
8) life stock: what fish are compatible? what fish should be stayed away from? what fish are good to start off with? which fish can you keep together and with corals, etc... what corals?
9) tank cycling
10) electricals and water
The biggest thing with marines, is that you require a LOT of PATIENCE!!!
Also: it helps to have a reasonable amount of disposable income.....
Remember: only bad things happen QUICKLY in marines....
i have done almost every hobby available to me and joined millions of forums but i gotto take my hat off to you okes this forum is well kept and the best maintained i have ever seen!!!! i'll buy you all a round..