Pure Sine is definitely a better choice for sensitive electronics but for heaters and pumps its usually ok to go with modified sine inverters.The only time you may run into problems in a aquarium scenario is some fluorescent lights have trouble with modified sine. Pure sine inverters are very costly compared to their Modified Sine wave counter parts.
As for the Gel batteries. They really arent a good choice for backup power in aquarium systems,imo.... They are better at slow low draws...Economically your best bet is a deep cycle marine type battery or solar battery..These can be typical wet acid batteries. The next option up would be AGM batteries... Which are also lead acid batteries with glass mats.. these batteries are SLA BATTERIES...they are sealed and spill proof whereas the wet cells are not. Gel batteries are also SLA batteries with a gel matrix. They are harder to charge and maintain and not very popular.
info here on The difference between AGM and GEL BATTERIES...
https://www.batterystuff.com/kb/arti...el-vs-agm.html
hth,
Al
AquaticSuppliers.comFoods your Discus will Love!!!
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Al Sabetta
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Aquaticsuppliers.com
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I was simply looking for something that would keep the fish alive for 5 or 6 hours and at $23.00, I felt it was a great choice. I plugged my cascade 1000 canister into it to see how it would handle it. Well, it didn't handle it well. Sounded like the magnet was slapping the side of the motor. I have four tanks, so this will be moved to our Cichlid tank and I will get the APC 850va for the Discus tank. But this $23.00 UPS was well worth it.