As an avid swimmer and shark hybrid, swimming for fitness is not something you just do. Different strokes vary your results greatly, working different muscle groups and body systems. If you wanted pure fitness out of the endeavour then it comes down to how much time you have. Is there a local olympic pool? I prefer to talk in laps as 50-meter bites.
1) Cardio: Freestyle with three strokes to a breath to start, using tumbleturns to add an extra bit of strain to your workout. This is a great start to anyone doing casual swimming as it prepares you for any further, intense swims. Move up to 4 strokes to a breath if this starts to become easy, and when you can do 5 full, proper strokes to a breath you'll be kicking ass. A longer pool helps this. You'll notice results fairly quickly, and by fairly quickly I mean 'gradually over a few weeks'.
2) Weight-loss: Breastroke, actually. Anyone who doubts the power of this stroke has never really tried to pull their weight through the water with any semblance of power. This works for short pools and if you love to feel the burn then this is the stroke to get your heart going, add in tumble turns and 30 of these suckers going full force should make you sleep like a baby.
Steer clear of butterfly unless you think it's a cool thing to know, because it's a *****.
Really, I don't think a 30ft pool can do you much good, the break in routine so frequently will play havoc with your stroke (and hence potential benefit) and pace, break up your rhythym and breathing, and so many turns so quickly will tire you out before you've done any real work.
Start with a 750m swim in an olympic pool (50m), just freestyle, and learn your limits. From there just go for broke. I swim 2k regularly, in both pools and the ocean, and it becomes a second way of walking, you can get into rythm beyond the strain and aches, and it leaves you feeling like a champion. Nothing like it, really, I put it up with parkour and martial arts as things that are just cool to learn. And no, being able to paddle in your home pool is not swimming in the proper sense, any more than shadow boxing is karate or stepping over potholes is freerunning.