How much load weight can a 4x4x14 redwood beam bear
I could not tell from the OP whether the 4x4x14 was attached to the bottom or the side of the 3x3x14 beam. If attached to the bottom, the post regarding the need for proper shear connection between the two beams is spot on. If attached to the side, the connection is not as important. The moment of intertia of the composite section is completely different if the beam was attached to the side versus the bottom.
I also was not clear whether all of the joists were added on to in the same manner, or possibly only the one that is going to support the fish tank. Since the tank is apparently aligned along the axis of one of the joists, it may lie entirely on one joist, in which case it would have to be treated as effectively a point load. If it is a point load, the critical factor is the distance from the support ends, along with the weight. Once we know the method of construction of the beam, and the location of the tank, we can compute the bending stress and estimate the factor of safety against failure.