This isn't for you as much as for some someone looking on, but also look to see what the floor will hold. Water, at almost 9 pounds per gallon, is just the start. There is also the stand. Gravel, decorations and accessories are denser than the water and add more weight. (This is the one time I'm glad we have a slab foundation and We still tend to put tanks close to the wall.)

You also don't want it where drafts, really warm air or the sunshine will hit it sometime during the day. Not such a big deal with one tank as in the fishroom, but if it is not easily accessible, it will not get as much care as if it were very accessible.

Something much trickier with marine talks is leaks and splashes. "Salt Creep" just kills furniture and floors. You can imagine what else doesn't need salt water on it.

Any big tanks need more care in leveling them. The stand can be "shimmed" with little pieces of carpet samples matching yours. If you would prefer to shim on top of the stand, cut cardboard and add about twice what you think you need, to compensate for compression. A small carpenter's balance (or a large one borrowed from the handy person on the block) is essential. This all helps guard against "Salt boom".

There is a TFH (Tropical Fish Hobbyist Magazine) column called The Salt Creep. I enjoy the humor and attitude of someone who doesn't mind the double entendre.

If you have very hard tap water, you may want to investigate a R.O. unit. A common problem is the presence of micro algae, especially encouraged by minerals in the tap water. By mixing the marine mix with largely demineralized water, one can limit the phosphorus in the water and hence the unwanted algae. The retail price of R.O. units has really come down in the 19 years since I bought one. That probably is because so many more are being purchased. The bulk of those purchases are by health or cuisine conscious families and restaurants and even bottling companies, virtually all of whom remake their water. Another factor in the growing R.O. market is probably the dramatic growth in the number of marine aquarists. One can also arrange, sometimes with the flick of a switch, to save water for the family's cooking and house-plants.

Just grist for the mill!

All the best!

Try for the 70 (264 Litre) tank rather than the smaller. It's the same as for freshwater, the bigger the tank the more time you have to find and correct problems. I still haven't worked out how you know how many fish you can have with saltwater. I read once that it's about an inch per 5 gallons (a centimetre per 7.5 litres). You will probably find though that you don't really want all that many fish because there are plenty of other things to look at.