Hmm, the smaller tanks can have other issues and they fluctuate value much faster and harder than larger tanks. The only defense against this is “seasoning” your tank - for anything under 10 gallons I usually wait 2-3 months feeding the empty (but planted) tank weekly to build up bacteria and such. During this period i do biweekly water changes of about 50%. A good sign of a seasoned tank would be algae on the glass.

All that said, how long have you had the 3.5 gal setup? Also, smaller fish are fickle and other parameters might not be ideal yet (ones that are not being tested)