Best way to accumulate heat for evening

I have a large aquarium in my rented apartment.  It is a first floor apartment so I have a covered patio in the back.  In my attempts to learn about solar energy, I would like to make a hot water heater to keep the water in the aquarium at about 28 degree - the aquarium is about 300 liters.  I was thinking about using some fresnel lenses to heat an intermediate liquid (anti-freeze?) and storing that in a heavily insulated tank, then pumping the aquarium water through a heat exchanger in the tank to heat the water.  Temps here range from a low of 6 degrees in the winter to a high of 40 in the summer.  Obviously, I will have a different problem in the summer! Any ideas?

Hi, Tom

I was wondering if something like this might work:

Zeek said, "I have about 300’ of 3/4” poly pipe just sitting on my black shingled roof.  I have the water circulating when the sun is out with a pump in the basement by the hot water tank.  It does enough to get the water to 125f.  ...… just use the sun to heat water

source