Heat loss is about surface area, temperature differential, and conductivity. Sizing a heater is really hard, especially without knowing those things. There are a few calculators online, but putting in the same parameters will be like +\- 50% range, just because they use the same math but different assumptions.
Insulate any non viewing sides, keep the room temp up and tank temp down, and keep it as covered as possible.
The inline heaters are really expensive. I'd personally just do 3 or 4 smaller heaters and put them on a controller. If in tank put a guard on them.
Looking forward to your build.
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