no wait. from were did you get the 800W heaters?
it is not worked out that way!!. so if your aquarium will be let say 600 litres, then roughly you will need a total of roughly 2.9 to 4.8KW per day (1.6 to 2.6KW per day if you insulate the back, sides, top and bottom with 5cm of jablo) so to get that amount of elecricity from photovoltaic panels in malta, a total of 240W of panel will give you not more than an annual average of 1KW per day. if you fix the panels on a solar tracking device, you can then decrease that to 170W of panels to get 1KW per day average annualy.
now to use a solar water heater, then it goes something like this:
every 1 square metre of normal panels (not the syphon tube system) will give a maximum total of 600-700W of heat every hour for a maximum of 6-8 hours in peak of SUMMER. in winter (when you actually need heating) you can expect a best of days to give you 400-500W of heat per hour for not more than 5-7 hours, and an average for peak of winter will not be more than 300W per hour for a period roughly not more than 4 FULL hours.... so this will mean that you will need to have a slightly big panel to make sure that in winter you are covered with heating.
also to remember that there is a lot of plumbing and moving parts involved which WILL go wrong some day... may be an easy task or a tidal flood of water.
with photovoltaic you need to apply with enemalta for a 2 way electricity meter and sell the electricity to enemalta as it is not worthed to use a battery and inverter. the cost to purchase and maintain these things will be much higher... plus you can get a rebate if you go with a grid-tied system.