All metals have an oxide on their surface, a very thin shell around a bulk metal body. Dissolution and corrosion occur at this interface. Depending on what ions and acid conditions the metal will dissolve via the oxide or via the bare metal if the oxide have been compromised (oxides are a passivating layer - stopping corrosion).
In truth, there is such a small amount of lead corroding at a very slow rate, and regular water changes should help keep the levels low but a tank is an otherwise closed environment and the living body cannot excrete lead so any contamination just accumulates, however slowly that might be. Regardless, there is quite a it of research out there debating the detrimental effects of lead poisoning in wild aquatic species due to lead deposits from lost fishing weights, bullets, paint, etc. Remember the Siamese fighters ill from being kept in lead crystal bowls?