Pianists don't need special exercises or devices to strengthen their fingers. First, your fingers don't have any muscles. Your fingers are moved through somewhat of a pulley system. Flex your forearm muscles (flexors on the belly side of forearm) and they pull a tendon which is attached to your finger bones in various locations. The muscles on the top of your arm are your extensors and their tendons run on top of your hand and fingers and straighten those out. These muscles also move your wrist. Think of your arm as being robotic.
Strength and power in playing come from gravity or the weight of the arm. If you are pressing into the keys with your "fingers," you will inflame your long flexor tendons which will then press on your median nerve (carpal tunnel syndrome).
I don't know what you mean by space between your two fingers. You don't want space between any of your fingers when you play. First, the hands/fingers can only go in one direction at a time and when you abduct your fingers (spread them out) you create vector pulls (uneven playing/illusion of weakness) because your hand/arm is being pulled in two directions simultaneously. This results in uneven playing and the illusion that the fingers are weak. They are, they have no muscles. When you walk, how hard do you have to press into the ground to walk effortlessly? None, gravity holds you down. It is our up muscles which make walking effortless. That is why walking downstairs is easier than walking up: Gravity. The piano is the same. There is never any reason to press into the keys or require more strength. This is why 5 year old prodigies can play better than most of us, they have mastered gravity and weightlessness (which is not the same as 'relaxation.'
Also, when you abduct and flex at the same time, you are using two muscles to move one bone in two directions at the same time and that results in the tendon getting pulled in two directions resulting in uneven playing but also will inflame your tendons giving birth to median nerve entrapment (carpal tunnel syndrome). Here is a video which sort of explains it: