So really quickly, needed to find out if this would even fly as an idea. Wired a TCS34725 up to an ESP32, and an OLED, and taped the sensor to the cartridge at different points. I have a one that is nearly depleted, so it's an easy test. I think it works more or less...
I can see a difference in the RGB channels at various points. The definitely dead side is different than the brand new one. Mind you, just taping the thing on isn't great, because there is no shielding of light. I'm curious if the fact that the container is clear will mess up the readings significantly. My idea is a strap 3d printed that will slide on, and try to shield ambient light as much as possible. Probably needs to be printed in black, and have some extra width to provide more cover.
For the other sensors, my thought is a BMP280, to gather humidity. Because the IO breakout on the neptune is just on/off, the best I can do here is either a high/low, or maybe if I eat 2 pins, I can do 2 bits of precision. Like 20% == 0/0 40% == 1/0 60% == 0/1 80% == 1/1. I might change that up, maybe range from 30-80 or something... But then you could code reactions at different levels, like if it's 1/1, then alarm, because it's stupidly humid in your fish room.
Because the Apex needs simple switched inputs, I'm using a relay board with the normally closed side to provide the switching on the color sense, and the humidity. Basically I'm thinking open for resin OK, closed for replace.
I think the only way to really do this is to print the mount and test for real. I think I'll do some more quick tests, and then design a circuit, and order a PCB. I refuse to jumperwire ever again.