If I may.
Seems Micky Q is referring to the 'Gels' that they use in the lighting industry as filters - i.e. these are colored sheets of Mylar or other plastic. In this case the sheet would be effectively a Blue Spectrum Pass filter. I.e. it would filter most other wavelenghts out of the incoming light only allowing the Blue wavelengths to pass through.
Net effect - Viola - Blue light.
Seems this would be just fine assuming that you are doing this for appearance only (seesm that way cause you only mentioned fish in your tank - i.e. no coral no live rock etc.). I mention this because not only do those organisms need the blue light spectrum but they need a reasonable intensity of that spectrum as well. Standard flour. bulbs have the blue spectrum but only at low intesities. This means that you will not get very much light out of the fixture once you put the "blue gel" on it since you will be filtering out the wavelengths that it produces best.