Planted Tank Guru
Join Date: Nov 2009
Location: Missouri united states
For juvenile Oscar, Fifty five gallon's would work for maybe eight month's.
Healthy Oscar,other Large cichlid's grow on average about one inch per month.Considering the juvenile's sold, are usually around three inches,,the fish could (with good care) reach twelve inches inside a year.(Is not uncommon)
Fifty five gallon tank is only twelve inches front to back and adult fish would not be comfortable (could barely turn around).
Were it me,,I would use large gravel substrate no more than and inch deep to make gravel vaccuming more effective considering waste out put from the Oscar and turtle which I might re-consider.(deeper substrate allow's more organic's to become problematic)
Plant's would be largely anubia 's attached to wood,along with maybe java fern.
Plant's in substrate are not likely to last long with the Oscar and or turtle but perhap's a sword plant or two in a pot with soil, and gravel cap might work.
Both of these creatures can and often do,,re-arrange the tank's bottom to suit them.
Filtration for the 55 gal would for me,,,be filter capable of moving 4 to 5 hundred gallon's per hour with twice weekly water changes once the Oscar reached six plus inches.
Hope some of this helps.