Hello! You would be better using ammonia and doing fishless way,leaving water in tank for 2 weeks wont make it redy for fish,the fish will sufer when they go in from poison
Not a good idea to put betta with other fish,he will kill your other fish or the danio will shred his fins. red lines need a bigger tank than 70liter,they need 4-5 feet tank for good swiming space.
Tank is not big enough for clown loach,he should be kept in group of 3 or more and a 5feet tank really. Tank looks nice, fake plant is very really looking,nice. You need more plnats and it will look good.
I have baby corys that have been in the tank from eggs and have grow up in that tank no problms,if there are no otherfish that would eat them they should be fine. Mine eat what the adults eat