Does anyone use chemicals in addition to a water filter?

No, as backup but not at the same time. Redundant. For me the entire purpose of the filter is to NOT endure the wait time while the chemical performs it’s thorough process, which a half hour to 4 hours, depending on water temperature and organism of concern.

“ Treatment Time
All chemical treatments take about the same amount of time. To start the process, you need to activate the liquids by pouring a few drops of bottle one and two together in the mixing cap. This activation takes five minutes. Then, once you add the activated chemicals to your water, you must wait 20 to 30 minutes for sterilization. The more turbid (or murky) the water, the longer the process will take. According to Aquamira, different organisms take longer to kill. If you are worried about Cryptosporidium, a particularly resilient protozoan, you will have to wait up to four hours. For viruses, you should wait 40 minutes. Bacteria should take the allotted time mentioned above.”

www.outdoorgearlab.com/reviews/camping-and-hiking/backpacking-water-filter/aquamira-water-treatment-drops

I’ve had a clinical lab diagnosed case of Giardia so I’ll carry the “extra” weight.

I’m curious what you expect the iodine to neutralize that the filter won’t block? Cow pond water will have gut E. coli from the cows or other livestock but they’re big enough to be filtered out (and aren’t pathogenic anyway).

Sometime in the recent past a woman turned up missing in Zion. Days later she popped back up and when the sheriff asked her how she had survived she told him by drinking Virgin river water. He was very skeptical saying the Virgin was full of Cyanobacteria and that would have quickly killed anyone partaking of it,

Cyanobacteria can be filtered but that doesn’t remove the toxins. Boiling only ruptures the little bad boys so a bigger dose of toxins. I’m not sure if it’s easily purified either after speaking to educated folk who have tested and found the stuff. Filtering might be better than nothing if one is dying of thirst but iodine or other chemicals would just add to the flavor of the toxic concoction. It just depends on what is in your specific source so personally I’d avoid that end of the cow. Lol

There certainly is a lot to consider with water sources. We have the warm conditions for Cyanobacteria and certainly lots of mines scattered about with those warnings. We have more than 500 nights in southern Utah so it was constantly an issue. Finding a seep was the best we could do on every trip. That could be comical at times trying to reach it and get the water into huge Drom bags.

To the OP question: yes I carry purifying tablets on every trip and in my day pack. For the last few years, enough for 2-3 days water in my first aid kit. I use both the pump MSR Mini-works and squeeze type filters like Sawyer. Select the type depending on the planned water sources. Rotate out the unused tabs every two years. it is a hedge against filter fail and i have filled a few times of some ranch hose and decided to insure my gut safety and treated with tablets.

Cyanobacteria is new concern in the Mountain West. I love hiking the sagebrush steppe in my area but heavy metals and alkali is a concern. Fortification Creek WSA is one of my favorite areas but all the beautiful surface water is not potable. Along with my purifying tablets I carry some Ph strips to test.

I'm late for the party, as usual, but....

Iodine kills viruses, filters do not. The CDC recommends filtering AND chemical treatment as the safest way to drink water, just as safe as boiling it.

Here is the science
www.cdc.gov/healthywater/drinking/travel/backcountry_water_treatment.html