It’s not only the space they need. Don’t forget about the need for food and a constant supply of pods. Valenciennea are one of the few genera I find much harder to keep than a mandarin. Not only are they active and need room to swim but they also need a constant supply of food. I often recommend a 4-5” deep sand bed in a tank that’s 2 years old at a minimum, 5 years is best. These gobies often starve to death in captivity after wiping out a pod population so that’s where the 4-5” sand bed comes in. A 4-5” sand bed will most likely only be touched from the top layer, the lower layers will have the pod growth going on and will most likely be able to start that population. The 5 year old part comes in when you need a established and healthy pod population that doesn’t face mass die offs. You then also need a 2.5-3’+ long tank IMHO due to the activity they have but also so that the pod population can’t be killed off instantly.