Small reef tank chemistry

[Erink]

Hello! I heard you're the guy to go to for chemistry questions.
I have a 36 gallon reef/invertebrate tank (no fish) and I was told that the smaller the tank the harder to maintain steady parameters. I am definitely having trouble. Everything is all good then a few days later something spikes or is too low. I do water changes once a week, unless everything is perfect. I check the salinity and top up every morning. I test at least 2x a week right now.
Any advice would be greatly appreciated

 

[Spare time]

What are you testing? What kinds of corals do you have (aka sps, lps, softies)? And what is spiking or dropping for you out of what you are testing?