There was no need to empty the tank. What you had was an algae bloom and, although it looked unsightly, the fish would have loved it!
A shift in the natural tank balance would have caused the bloom. This could have been from a number of sources; the filters blocked a bit, more natural light on the tank as summer approaches, a bit of overfeeding, building phosphates etc. Perhaps, as the fish have grown, their wastes are now too much for the current filtration which is why the tank was fine until recently.
I like under gravel filters and find them good but would not use them as the only filtration in a 240L tank as I think it would not be enough. There is supposed to be muck under the plates, it is full of the good bacteria that makes them work.
There was no bug so no need to get new gravel but, as you realise, you are now going to have to cycle the tank again.
If you get another filter (eg an external or internal canister) run it for a couple of weeks on your other tank to start the bacteria going then you can transfer the filter and the fish back over to the new tank again.