would i be able to keep a terrapin (i dunno what species) in a 18in by 1ft x 10in tank

You could raise a couple mud or musks in such an enclosure but they would probably do better in a larger enclosure as they got older.

Ed

only for a little while, they get big fast, do loads of research

I agree with EJ, Musks or Muds only, no more than two, and probably for no longer than 6 months or so. For a pair you'd eventually be looking at a 250-300 litre tank.
Any other species would be too big for such a small tank even as hatchlings.
My single 6 month old Musk is already in a tank twice the size of yours, and I'll be upgrading that before the end of the year.

__________________
2.3.0 Southern Painted Turtles
1.0.0 Florida Red Belly
2.1.1 Common Musks
1.0.0 Classic Cornsnake
0.0.2 GALS + infinite babies
0.1.0 Tibetan Spaniel
1.2.0 Brahma Chickens

You might want to mention to the OP that your suggestion is the best you are providing. What would you consider adequate?

I wouldn't expect these turtles to double in size in a year. I have Chinese boxies that have not grown 5% in a year... that's one extreme.

Ed

For a pair of adult Musks I would consider a 250-300 litre tank to be the minimum adequate size, assuming they both grew to 4" or so I wouldn't go any smaller than that. That's not exactly an enormous tank in turt terms!

I'd expect hatchling Musks to double in size in their first year, with the growth rate gradually reducing as they approach their full adult size. Musks are slow growers compared to some species, but they still have that initial spurt in the frst year or two.

__________________
2.3.0 Southern Painted Turtles
1.0.0 Florida Red Belly
2.1.1 Common Musks
1.0.0 Classic Cornsnake
0.0.2 GALS + infinite babies
0.1.0 Tibetan Spaniel
1.2.0 Brahma Chickens

For the OP... I tend to respectfully disagree with Graham on this. You could easily house a pair of adults in an aquarium half that size.

Ed

My recommendation is based on the fact that Musks spend a lot more time in the water than most other species, they are very active and spend a lot of time rooting around looking for food.
They can also become quite aggressive towards others of their own species, so a tank which allows them plenty of personal space I would consider essential.

__________________
2.3.0 Southern Painted Turtles
1.0.0 Florida Red Belly
2.1.1 Common Musks
1.0.0 Classic Cornsnake
0.0.2 GALS + infinite babies
0.1.0 Tibetan Spaniel
1.2.0 Brahma Chickens

I have a 120L tank. If a musk only grows to 4-5inch as adults, then that is big enough. My tank is 40"x1'X15" and 1 or 2 4" turtles arnt exactly going to need that much room are they. So 250-300L is far over the minimum required.

I agree with Ed on this occasion.

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You're missing something though, it's not just a question of the turts having enough space to swim and move around, it's also having sufficient water volume to dilute the turt waste so that the filter can deal with it effectively.
Less water volume means a higher concentration of pollutants, and then you need an even more efficient filter to deal with it adequately, dilute that same waste in a lot more water and the filter's job becomes a lot easier.
The vast majority of health problems in pet turts are caused by dirty water, not only do you need a decent filter to keep it clean, you also need to have enough water to give the filter a fighting chance of doing it's job!

__________________
2.3.0 Southern Painted Turtles
1.0.0 Florida Red Belly
2.1.1 Common Musks
1.0.0 Classic Cornsnake
0.0.2 GALS + infinite babies
0.1.0 Tibetan Spaniel
1.2.0 Brahma Chickens