Honestly try to just ride it out. I just started a 20 gallon tank about 4 months ago, so I'm around the same stage you are. I've been battling diatom blooms for the past 2 months, but from previous experience this is just a growing pain. Turning out the lights for a day helps if it's gotten out of control, but besides that just get a good mag float and clean the glass daily. Clean the sand bed with water changes.
One thing I've recently tried is LESS water changes. I was doing about a 25% water change weekly, and would always have a big diatom bloom afterwards. I was away last weekend and didn't do a water change, and the diatoms have actually slowed a lot, like almost negligible. This could be a caused by a couple of things;
From what I've read Diatoms need silicate to form, and every time I do a water change i kick up a whole bunch of sand. I probably could have been way more vigilant about rinsing the sand before i started the tank, so every time I'm trying to clean the tank I'm actually kicking up a bunch of stuff into the water column including a bunch of silicate from the sandbed (correct me if I'm way off here).
Also, I've got an RODI that's always read 0 TDS, but it's probably 5 years old and likely the DI resin has expired. I've read that with the DI expired you could be adding a bunch of silicates to the tank through the water change and top off water. Just something else that might explain why i always have a diatom bloom after a water change.
Either way my advice is not to do anything crazy at 4 months because you're having a diatom bloom. Don't start dosing vodka or doing anything drastic. Obviously a bigger skimmer always helps, but maybe just try lights out for a few days and pushing the water change for a week and see what happens.
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