Of course the elephant is not really convienient for bigger tanks :dry: – I use it mainly for the 12 liters. But the principle of holding the hand can also with some effort be used even if using a bucket and a hose.
But I also do not change 50 % each time, – I change maybe 2 elefants 🙂 … (that would be around 3 liters)
Some plants needs time to adjust if they come from hard water to the soft acidic water in our parotanks – the ceratophyllum demersum sometimes drop all the ‘leaves’ – but it will ‘toughen up’ after some time.
The one you mention – ceratopteris – is one I also have problems with, some survive, – most of them dissappear. What makes the difference I am not sure.
I must say I rely mostly on javamoss and javafern and floating plants such as salvinia and duck weed :).