Holy moley, 1.028 to 1.024 (1.025) over 45 minutes? So basically everything is going through the same acclimation (e.g., a 3 point difference) that they probably went through when they came home from the store .... except that this is happening everyday.
I'm sorry but it can't be good. You should look at a slow drip system, float valve or float switch for topup. That way your SG will never vary .. or at the least it will vary imperceptibly. A slow drip may not be 100% match the rate of evaporation but if it can help you to make only minor adjustments on a daily basis (i.e., add only 1/4 - 1/2 gallons instead of 5).
Whether organisms can withstand varying salinities like that is going to depend on (among other things) where they live naturally, i.e., river estuaries, tidal pools, etc. will experience SG changes over a daily period but deeper water will be (usually) very steady. What we keep in our tanks tends to be a hodge-podge of animals from all over, so it stands to reason that some will withstand it, and some will not.
1.028 is a little too high. A SW fish can't drink water if he gets thirsty, they have to osmoregulate. So saltier water makes them have to work harder to keep from dehydrating.
BTW, 5g is a LOT of of topup for a daily addition. I'm going to assume you have either a large system, or at least a system with a reason for accelerated evaporation (e.g., halides and/or fans). A setup like that surely has a sump. If that's the case ... drill it for a humidifier float valve ($10 at Home Depot/Rona/etc.), use a 5g bucket for the reservoir, and fill the bucket once per day. Thus YOUR routine doesn't really change, but your tank will have steady SG over a 24hour period ... this will have a profound difference on your tank and how well it looks (really). And what's even better is it costs you less than $20. If you really want to geek out then you can look at float switches and whatnot ... or if it sounds too complicated then at least consider the daily drip thing.
Anyhow there's my $.02 on the topic ... HTH and good luck.
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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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