Hi amyherrera
Welcome to TFP
By a "15/42" pool, do you mean dimensions (length & width) of 15 feet by 42 feet? To properly dose any chemical, you need to know how many gallons of water are in your pool. The dimensions are one input in determining water volume and the average depth is another. Here's a site that I have found to be helpful in determining the pool water volume and it covers most pool designs:
http://www.poolandspachemicals.co.uk/volcalc.htm. Once the water volume is known, then follow the dosing instructions on the label. If you just dosed by "eye-balling" it without knowing how many gallons of water are in the pool, then it is hard to say if what you put in is too much. This applies to any chemical, not just algaecides.
With all of that said, algaecides are generally not needed in a pool if it is properly chlorinated according to your CYA level. One exception to this guideline is if you will be out of town and the pool will be unattended. You still need to make sure it is chlorinated, but a 60% Polyquat-based algaecide can be used in that situation to provide some extra defense against algae. The active ingredient for a Polyquat-based algaecide is:
poly [oxyethylene (dimethyliminio) ethylene (dimethyliminio) ethylene dichloride]. Look for a 60% concentration of that ingredient.