What are the dimensions of the tank? Some are 3' long whereas others are 4' long, with the 4' long size 40g's you could probably go with some of the 8" size eartheaters that aren't on the list but are in the link.
Smaller fish like Apistogramma spp., rams, keyhole cichlids, festivums, more angels, dwarf acara's, smaller species of eartheater etc.
List of things you could probably try (finding them all could be tricky).
Mesonauta festivus
Mesonauta guyanae
Mesonauta cf. guyanae "Rio Cobra"
Mesonauta cf. guyanae "Rio Tapajos"
Mesonauta insignis
Pterophyllum leopoldi
Pterophyllum sp. "Rio Momon Red Spot"
Pterophyllum scalare
Cichlasoma dimerus
Crenicara punctulatum
Dicrossus filamentosus
Cleithracara maronii
Laetacara curviceps
Laetacara dorsigera
Laetacara thayeri
Nannacara anomala
Apistogramma agassizii
Apistogramma atahualpa
Apistogramma baenschi
Apistogramma bitaeniata
Apistogramma sp. "Black-chin/Schwarzkinn"
Apistogramma borellii
Apistogramma sp. "Breitbinden/Broad-banded"
Apistogramma cacatuoides
Apistogramma cacatuoides (Orange Flash)
Apistogramma cacatuoides (Red)
Apistogramma elizabethae
Apistogramma cf. eunotus "Orangeschwanz/Orange-tail"
Apistogramma hongsloi
Apistogramma macmasteri
Apistogramma sp. "Mamor"
Apistogramma sp. "Maulbr"
Apistogramma nijsseni
Apistogramma panduro
Apistogramma sp. "Papagei/Parrot"
Apistogramma cf. resticulosa "Mamor"
Apistogramma rupununi
Apistogramma steindachneri
Apistogramma trifasciata
Apistogramma uaupesi
Apistogramma viejita
Mikrogeophagus altispinosus
Mikrogeophagus ramirezi
Taeniacara candidi
Biotodoma sp. "Aripuana"
Biotodoma cupido
Biotodoma sp. "Guyana"
Biotodoma wavrini
'Geophagus' steindachneri
Geophagus sp. "Araguari - Orange Head"
Geophagus brokopondo
Geophagus grammepareius
Geophagus sp. "Pindare" (Parnaiba)
Geophagus taeniopareius
Geophagus sp. "Tapajos Orange Head"
Guianacara owroewefi
Guianacara sp. "Red Cheek"
Guianacara sphenozona
Gymnogeophagus gymnogenys
Gymnogeophagus labiatus
Gymnogeophagus meridionalis
Gymnogeophagus rhabdotus
Gymnogeophagus sp. "Verna"
Apisto's and rams
Angels and festivums
Dwarf Acara
Checkerboards
Eartheaters