Use past tense to describe events that have happened. Such events include procedures that you have conducted and results that you observed. Use present tense to describe generally accepted facts.
We sought to determine if mating behavior in Xiphophorus helleri is related to male tail length by placing combinations of two male fish with different length tails in the same tank with a female fish.
We found that protein synthesis in sea urchin embryos treated with actinomycin D was considerably less than in untreated embryos. This finding agrees with the model stating that protein synthesis in 24 hour sea urchin embryos is dependent on synthesis of new messenger RNA.
Reference to results of a specific study should also be in past tense.
Abercrombie and Fitch reported that 30% of the public is allergic to wool.