I too have an Intex pool. Mine is 15' diamater, 3.5 tall. This is the second year for our pool Last year I kept the same water in if from April to October. This year we set it up in April as well, but didn't start swimming in it until May. I don't use a chlorine system at all. My DSis has had a jacuzzi for 7 years now and they use the Baqua system which is a chlorine free methed. She loves it so we decided to use it on our pool.
Last year I spent less than $75 on supplies and the pool worked really well. Only had it green up once but that was my fault, when it got realy hot I didn't take of the solar cover and it reached over 104 on the top! I was able to get rid of the algae by running a second pump (from a previous pool) and it got rid of it.
This year, I've had a little more trouble but I'll be honest, I haven't been running the pump everyday.

And we've had more swimmers in the pool than last year. The neighbors have been coming over about once a week for a swim. I noticed an increase in, how should I say this... there's been more stuff floating on the top-hair, skin, hair product, etc. When I'm in the pool I'm meticiulous about skimming out debris. Even with the cover on all the time, we still get bugs, leaves, etc. in the pool.
I took my water to be tested yesterday since it was cloudy. (This is the first test I've done all year!

) I was low on Shock, low on calcium (this is why it's cloudy), and needed Aglacide (sp?). I treated the pool this a.m. and added the second pump (and then realized that I've had the smaller pump on all summer-need to switch them). The pool is looking cleaner and there is less algae. I've been pushing the algae and stuff around on the bottom to get it to the top so the filters will pick it up.
The main problem with these pools is the filter system just isn't that good. For my pool, it looks like adding the calcium stuff has made it clearer. The Shock and Algacide should take care of the slight green issue. I just need to run the pump everyday, as required.
I really like the Baqua system (there are also other brands). I only have to treat the pool about once a month. There's no chlorine to burn the eyes and bleach the hair. And I don't have to test it as often as the chlorine system recommended. (once a day and after every swim session).
I also love our solar pool cover. We bought it last year for about $50. It looks like blue bubble wrap. The pool stays warm and can be as much as 10 degrees over the outside air. Plus it doesn't lose the heat at night. During the heat of the summer I have to take it off during the day as the pool gets too hot. It can be well over 100 on top in August/Sept.