Here's a grating for the top of SnowMan77's great large tanks (https://www.thingiverse.com/thing:2319085). I made this so that figures could stand on the tanks for a second level of height. Printed with an Ender3 at 0.12mm, no brim, hatchbox silver PLA, Cura. You might need to adjust your inner wall thickness and/or fill-gaps settings to get it to print smoothly - check the slice previzualization.
Edit: Rotated the grate 180º.