Have you looked into using a Ph sensor? Search on "arduino ph sensor"
I suppose you are not sticking the temperature probe in the water? Search on "arduino waterproof temperature probe". Though they are easy to make.
How about auto aquarium light management?
Outside canister filters? Flow rate readings. Useful to know when its time to clean a filter.
I'd not use a UNO for this but you could have the micro controller publish the results to a MQTT broker and from there you can set it up so that you can read the values from a deserted island in the South Pacific. Heck, you can even control things. Example, you are away, you see the ammonia level rise and remotely fix it.