I have a 20 bucket setup for tomatoes, capsicumes, chilis and cucumbers. All going well but as weather has got warmer has become very clear need a bigger reservoir to compensate. I know way too small but only around 40 litres currently.

Unfortunately the design of my system has been limited by both funds and being a renter. The system is down fairly though and drains via PVC along each side to the current too small reservoir.

I have a 100+ litre tub available which will be great, about bath tub sized. I'd like to use the current small tub as a sump and pump from here to the 100+ litre and then pump to the plants from here.

Currently I pump to the plants 4 times a day for 30 mins.

But I'm really lost as to how to set this up. If I just put a small pump in the sump will be sucking air most of the time. I can't figure how to setup a float to do this, obviously you do the opposite normally; turn off a pump when the float reaches a point, no the other way around.

Is there such a thing as a pump that can "detect" when water is at a certain level then run for a certain amount of time? I know in the US they have "sump pumps" for their basements, but these look very very overkill for what I need. If i can somehow setup one of the pumps I have now to do this would be great.

Any ideas at all appreciated.