I have only gotten a flashing "Lo" since plugging it in. What am I missing from the limited instructions?

Honestly, I have no idea. The instructions are minimal and there is almost no documentation online. All I know is I set it and forget it and it keeps a perfect temp.

Which wattage should I use for a 66 gallon aquarium?

I am using 300 W for my 60 gallon tank.

can i hook this up to a eheim 1500xl?

I don't see why not, it's just uses your standard tubing, cut the current tubing and put the heater on the outflow that goes back into your tank. Mine works perfectly, however it does read only in Celsius, so you may need to make that conversion to Fahrenheit to dial in the temp. Just use google for that.

Is there a reason as to why the inlet/outlet ports have changed? I ordered the 500w 16/22, but seems to have both sizes on the ports with no lock nuts

Yes they are the same on both ports, didn't pay it no attention but they don't come with locking nuts, because I used a couple of clamps to hold mines on....Pros: A single 500 watt heater actually does a good job on heating my 220 gallon aquarium to desired temperature! Cons: with it being connected to the outake of my eheim 2262, it reduces the flow tremendously!!!

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ISTA I-H616 500W Inline External Heater

Product ID: 43163894

Details

Brand
ISTA
Power Source
Corded
Color
Black
Wattage
500 watts
Voltage
230 Volts

⚡500W powerful heating

🌡️Dual temp sensors for precision

🛠️Compact external design

Description

🔥 Keep your tank perfectly heated, no fish left behind! 🌊

CLEAR CELSIUS DISPLAY - Easy-to-read temperature shown in Celsius for professional monitoring.
RELIABLE POWER OUTPUT - Consistent 500W heating power ensures your aquatic environment stays perfectly cozy.
RAPID HEATING EFFICIENCY - Thermal sprayed glass accelerates heat transfer, warming your tank faster.
SPACE SAVING EXTERNAL SETUP - Mounts outside the tank, freeing up valuable aquarium space and simplifying installation.
PRECISION TEMPERATURE CONTROL - Dual sensors monitor inlet & outlet temps for ultra-stable aquarium climate.

The ISTA I-H616 is a 500W inline external aquarium heater featuring dual temperature sensors for precise water temperature monitoring. Its thermal sprayed glass element enables faster heating, while the external design saves tank space and simplifies installation. Displayed in Celsius, it’s ideal for both freshwater and saltwater tanks up to 125 gallons, delivering reliable and consistent heat output for a stable aquatic environment.

Specifications

ASIN	B01MD1LR37
Best Sellers Rank	#332,765 in Pet Supplies ( See Top 100 in Pet Supplies ) #566 in Aquarium Heaters
Brand	ISTA
Brand Name	ISTA
Color	Black
Customer Reviews	3.7 out of 5 stars 97 Reviews
Global Trade Identification Number	04719856836164
Heat Output	500 watts
Item Type Name	ISTA I-H616 500W Inline External Heater
Item Weight	15.84 ounces
Manufacturer	Ming Trading LLC
Manufacturer Part Number	I-H616
Material	Glass
Material Type	Glass
Model Number	I-H616
Mount Type	Floor Mount
Number of Items	1
Power Source	Corded Electric
Special Feature	Dual Temperature Sensors
Special Features	Dual Temperature Sensors
Style Name	Indoor Installation
Unit Count	1 Count
Wattage	500 watts

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