hello! I have a nano tank rn 13g and like you I battled algae forever, finally I figured out how to beat it so let me reccomend what I did.
1. Up your flow
2. I would increase your 4g WC to at least 10g once the algae is gone you can work back down to 4.( in my 13g I do 5g ever 8-10 days)
3. Decrease lighting photo period (possibly intensity too)
4. Add seachem matrix/ marine pure in a high flow area
5. Vacuum your sand in very small areas to begin with
6. Snails snails snails a few hermits more snails, turbo snails, and a emerald crab or 2 ( I had a really good one that just passed ��, he eliminated my bubble, and the crew cut algae)
5. With the Phosguard the most important thing to remember is that you must change it out!!! I too didn’t change mine all that often but once I was determined to beat the algae I changed it every 4days then gradually every WC and now every 2 weeks.
Good luck!