agreed --- batteries are the way to go ... even without an inverter, you could run a couple of 55 watt halogen headlights off a 200 amp battery for about 24 hours (if my math holds up?
200 amp deep cycle X 12 volts = 2400 watts / 110 watts halogen light = ~ 22 hours
plus halogen put out a lot of heat - should take care of your heating problem
daisy chain a pair of 200 amp batteries, and you can double your light wattage, or double your run time
55 watt bulb and socket cost around $12 at an automotive store - sans the socket and it's even cheaper, just solder some wires onto the bulbs terminals
and this setup will put out a lot more light than 1000's of candles could ever hope to produce ... even a single 20 watt mr16 bulb will puts out like 1000+ candle power
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(begin Rant)
the facts are facts ... there is no "dirt cheap" method of suppling emergency lighting / heat or filtration unless you happen to have some or all of the parts that are mentioned in other posts
troutman's DIY stand-by UPS is a great plan, and if you had a few of the parts like he mentioned, it's very inexpensive and effective
building a 500 watt gas generator out of $60 worth of parts is another great idea, if you've got access to what would otherwise be "junk"
creating light takes a LOT of energy, which has to come from somewhere - stored chemical energy in a parafin candle is insufficent to produce any PAR light (photosythetically active radation) ... besides, the color temperature is like 1800 kelvin, most of that light won't even penetrate water ...
maybe you have some 24 hour emergency candles that are made from very hard wax ... so you rig up something that uses 100 of those ... unless someone's giving them to you for free, that sounds expensive - and you only get one day's worth of use out of them ... plus with only 100 candle power, it might as well be moonlight for the plants, they're not going to photosynthisize with that little light ...
stored chemical energy in a battery is much more concentrated, and it's a LOT cheaper per unit then a candle ... a $30 marine battery can run a pair of 2 million candle power (~200 watts ea) floodlights for a few hours ... thats a LOT of candles ... and with a $15 battery charger, you can use the battery again and again and again ... and adjust your "run time" by selecting the quanity of light you need to produce (4 million candle power is way too much
investing in a 600 or 1000 amp marine battery and a good inverter (dc to ac converter) would give you enough power to run 100's of watts of lights, heaters and filters for at least 24 hours, probably longer - add a charger and you're looking at under $200 for a system that will last you for years
an inverter will reduce the run-time of your battery, but would let you use existing equipment
as far as using exothermic chemical hot pads ... the reusable kind are expensive ... plus how are you going to transfer the heat to water? glass is a very poor conductor of heat, taping these things to the sides of your tanks will result in most of the heat being carried off by air currents caused by convection, and if they get too hot, it could cause a seal to rupture on your tank because one 'pane' of glass is 30 degrees warmer then the others it's joined to
if these heat packs are water proof (even if they're not water proof, sealing them inside a few ziplock bags shoudl work), submerging them is your best bet, since they will transfer their heat directly to the water, and create a very mild water current as well due to convection
sorry for 'goin' off on ya ... but this candle thing is just crazy in my opinion
(End Rant)