05-11-2013, 11:18 AM
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To your question on heaters. For a 4-foot tank, you have two good options. One is to get a canister with a built-in heating unit. The second is to have two heaters. One separate heater in a tank this size is not wise.
Considering the two heater option, you place one at each end, preferably beside the filter intake at one end and the filter return at the other [assuming a canister filter, which in my view is the best option for this sized tank]. I have this arrangement on my 115g 5-foot and my 70g 4-foot tanks. My 4-foot 90g has an Eheim canister with the heating element [this is by far the best method, but these filter/heaters are not inexpensive].
When it comes to buying heaters, always buy good quality. This is the one piece of equipment on which you should never scrimp on money. Filters and lights can fail and you have time to fix things. A heater failure overnight can mean a tank of dead fish, either by overheating or by failure and cooling; I've had it happen.
For a 4-foot 75g, I would get two 200w heaters; 150w could manage, but there will not be that much difference in price to risk it.