That depends on a couple of things. First off, what are the dimensions of a tank?
A 75 gallon is about 21" tall. There are also 80 extra highs that run around 30" in height. The depth will impact the amount of light needed. Typically the standard t8 strip light that come with kit tanks are inadequate, but there are exceptions to every rule.
I have a 90 gallon with 2x54w T5 HO GLO fixture on top of the tank above the glass cover and supliment that light with canopy DIY fixtures 2x65watt 10k's HO's without CO2. I dose Excell every other day with some trace and flourish 2 x's a week and water changes 1xmonth..tank is a year old and the plants are doing great. Very little algae now that I remedied most of the problems.