The container will, at 80l, be too small in a very short time, as goldfish grow quite quickly under normal circumstances. The pepper might be problematic as they like warm weather. However, it is worth a go. I grow rosemary in a pot and I would think mint could be grown the same way, so you could save the space for your other crops.
Yeah ur right about the rosemary and mint, though I may need a bigger pot for the mint. However, I'm going 2 try and grow a couple of (bell) pepper plants and 3-5 strawberry plants on the top shelf/tray, and 4-7 Lettuces, some herbs and spices on the 2nd shelf /bottom tray. If each pepper plant grows 7 peppers and each pepper takes 7 weeks to grow, then I will have 2 peppers each week, and 1-2 lettuces each week, which will be enugh for my family of me, my mum, my dad and my brother.
I'll keep with the 80 L for one common goldfish and 2 Hong Kong plecs for now, and in 3-5 (or 7+ if possible) years move them (the goldfish definitely) into a bigger container.
By next summer it should be powered by a tiny wind turbine made from pipes for the blades, and generators etc and have another but smaller container for growing algae, shrimp and duckweed. The duckweed will be the main source of food for the goldfish, the algae being the main food source for the Hong Kong plecs, and a couple of shrimp being fed every week or half week to all three fish. The goldfish may also have some algae and aquatic plants etc. Sometimes I'll feed the shrimp and the loaches, but mainly it'll be self sustaining.
1) The shrimp will eat algae (and some shrimp food)
2) Their waste is a fertiliser for the duckweed, and algae, and both types of vegetation will clean the water for the shrimp
3) The duckweed will be the main source of food for the goldfish (I will scoop it into the fish tank), with the shrimp occasionally and the goldfish can also eat their waste, algae and aquatic plants (also occasionally plants from the hydroponics)
4) The Hong Kong plecs will clean the tank of algae, being the main source of food for those two fish
5) The nitrogen cycle will take place and the worms in the hydroponics will eat any other fish mulm (or whatever it's called) and decomposing leaves, stems, roots etc,
6) The nitrates, the worm tea, phosphates and small amounts of nitrites and ammonia will fertilise the plants, and the plants filter this out making it clean for the fish