25ppm, not great but could be worse. Try the 50% change and see how the tank goes.
The problem with water changes as a means of nitrate control (or "management" if you prefer) is "diminishing returns." If you did a 50% water change one week, then your nitrates will be ~12ppm after that water change, so about a 12ppm reduction. Assuming no nitrate buildup and no other means of nitrate reduction, if you did another 50% change the next week now you only get a 6ppm reduction. And the next week only a 3ppm reduction, and so on. But the cost of doing that 50% changeout remains the same. That's why I feel when battling a tank "issue" you want to go as big as you feel comfortable with. Multiple small changeouts can't solve a spiraling problem (although they can be good preventative maintenance assuming everything else is in balance).
But .... for a 20g tank, water changes are less expensive compared to larger setups. A 50% changeout is 10g, and that's 2 5g buckets. Not bad. Gets a little more complicated when you're talking about using several 30g rubbermaids for one water change.
Curious though: Have you tested PO4?
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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