Although much attention has focused on self-motile asymmetrical catalytically active "Janus" colloids as a route to enable new fluidic transport applications, the motion of symmetrical catalytically active colloids is less investigated. This is despite isotropically active colloids being more accessible and commonly used as supports for heterogeneous catalysis. Here, we addressed this by systematically investigating the motion of platinum-coated colloids capable of isotropically decomposing hydrogen peroxide. We observed the onset of collective convective flow as the colloidal volume fraction increased above a threshold. The ballistic velocities induced by the collective flow were quantified by particle tracking and were found to increase with the volume fraction. We also determined the associated increase in the Péclet number as evidence of the potential to use convection as a simple method to enhance mass transport rates. By determining the persistence lengths, we were able to correlate the magnitude of convective flow with the overall catalytic activity per unit volume. This suggests that the mechanism for the collective flow is driven by chemical activity-induced local density differences. Finally, we discussed these results in the context of potential new fluidic applications and highlighted the role that activity-induced convection may play in experiments designed to investigate self-motile catalytic systems.