[wildreef]

Dh , your slainity swings can come from 2 things,

1, when doing water changes always pull of the same amount of water that you plan on putting back in.
( example: 5 gallons removed , replenish 5 gallons. )

2, when water evaporates from your total sum of water , Usealy noticible from the level in the sump ( i have a mark in the sump where i like to keep the water level )
(example: if water has evaporated from the sump, add back the same amount of RO/DI
water ONLY.)

Reason being the water that evaporates from your tank is plain water , the salt remains.
If you remove water intenionaly for water changes , you are removing salt along with it.

make sure when you do water changes that you check salinity first , if it reads 1.025
before doing a water change , then have your make-up water read exactly the same.
thereby keeping salinity the same as where you had it.

If you wish to increase salinity , then increase it,adding more salt to the make up water slowly raising it over the course of a few days.
If you want to reduce the salinity then you add less salt to your make up water by a couple of points ( over time and sevral water changes ).

Also making up your change out new water it's suggested that you make it then let it set over night or atleast a few hours to give the new water and the salt in your make up tank/bucket time to disolve.
if you added to much salt to your make up water , then it will be much easier to correct then and alot safer
either by adding water or salt to get the desired targeted salinity.

I dont suggest in making salinity changes in one huge water change,
Most fish don't like that and Corals definently don't.
Your best off keeping the salinity as stable as possible.