No problem about the comment, just thought Id let you know as some people are funny like that
Sorry to hear about your featherfin
How long has the tank been running for? you want the filter to turn over around 5-10 times the tank volume, so a 50 litre would need a 500litre per hour turnover.
Feeder fish are certainly not well accepted on this forum as there are many a gold fish fanatic that may take offence and reoport any user/comments. Besides the goldfish can carry diseases that may do alot of harm and no good to your fish. Spirulina flakes may help boost the vegy content in the diet for the oscar, all my cichlids love the stuff.
Check
HEREfor oscar food and care guide
The pleco will love the plants, pleco wafers, cucumber, corgette, lettuce etc
If your looking for a tank, and are in the UK ,
THIS WEBSITE has Cheap tanks, but are perfect as i have one myself, there imported from china but the company is a british one down south. I bought a 330 litre tank with hood, cabinet, T5 Lighting, Heater, filter, filter media, algae magnet, syphon all for £330!!!! Thats 4 Foot tank
Hope this helps