Do NOT use citric acid.

If you want to re-dissolve the iron, use ascorbic acid. But remember that NEITHER are compatible with chlorine.

You need to get clear on the difference between dissolving iron, and removing iron. The ascorbic acid will help redissolve iron where it has stained surfaces. But it will do NOTHING to actually remove it from the pool -- rather it just *moves* the iron from the stains ON the pool to the water IN the pool.

By contrast, the cal hypo will do NOTHING to help with the stains, but it will enable you to remove the iron that is IN the water.

The expensive of the cal hypo is not an 'extra' expense, like ascorbic acid will be. Rather, it will become THE way you chlorinate. The reason for using it is that WHILE it chlorinates, if you use it correctly, it will ALSO remove iron FROM the pool water and deposit ONTO the calcium dust collected by your filter. From there, both the dust and the iron will be actually removed from your pool each time you backwash.