The paired t-test would have 49 d.f., as would the error term of the randomized block design. Total 99 CRD with a priori treatment arrangement. Aquarium is both the sampling and experimental unit, where the 4 combinations of drug treatments A and B are the treatment. The effect of the different drugs can be tested with contrasts. Drug Combination 3 Error 12 Total 15 CRD with a priori treatment arrangement.
Plot is the experimental unit and the sampling unit.
Fertilizer variety 2 Error 6 Total 8 CRD with a priori treatment arrangements. Aquarium is the experimental and sampling unit.