I feel you about the tank size.
Try testing your tap water and test the water after conditioner (before adding it to the tank) to double check.
Always test the water you conditioned before adding to tank (ofc u need to wait 20 min before adding it to the tank) because sometimes the amount you are following is not enough or too much...
Guppy in my tank is impossible, they always die, they are not strong fish like livebearer. So the water parameters are the most important
Well, now, let's talk about the oxygen, u have a few plants that are taking nutrients from the water, co2
and others, so take in consideration that it could be damaging the parameters for your fish.
Keep cleaning your tank with weekly water changes and dont overfeed. 5 male guppy and 3 cory can be a lot for a siletstream filter to take, i have a 40g filter for my 20g tank and I still doing some weekly water changes sometimes.
Your tank is good, just check the plants roots, leafs, coloration...they also can demonstrate your water parameters harmony.
You will be good, maybe some improved gear for the tank