in the tank. they're installed in the previous pic. here's a couple close-ups.
you can see where i've assembled both sensors for the top-off and the temp probe from the reefkeeper into a single array. that's the ph probe on the far right. for the most part, the probe is pretty camouflaged. the only noticeable part is the bright tip. i've got my heater and grounding probe hidden in my skimmer.