looks like a pump then . i was trying to avoid it but it looks like thats it was worth asking incase someone had some new amazing idea . the drains are internal and sealed so they are a mare to get anything new into and draining into the garden isnt great as you end up with a soggy garden
p.s. i do have an existing floor standing tank and i have a pump free dranaige system on it. tank needs topping up every other day but it wont work on bigger tanks or tanks with an enclosed top but thats another story