It would be fine for dp's you could go for 3 if its 40l, there are a few other puffer species you could put in there like carinotetraodon lorteti, irrubesco etc. Imo halfbeaks are the ultimate under 3" oddball. There not easy too get hold of these days, celebes halfbeaks are available but aren't as small or brilliant as wrestling halfbeaks dermogenys pussilla