We are using buffered formalin for initial fixation (3-5 days), then change to ethanol. For fragile organisms, it is recommended to increase the ethanol concentration in steps (of a few days each), starting with 35% ethanol, then 55% ethanol, final and permanent storage in 75% ethanol.

Depending on the purpose of your sampling, deep freezing may be an temporary alternative to formalin/ethanol fixation.

There are a number of researchers that use buffered formalin with gradual ethanol replacement but I am guessing that you want to avoid formalin altogether.

Freezing is another option or using liquid nitrogen perhaps?

I also found this paper that may be of some use: Groelz D, Sobin L, Branton P, Compton C, Wyrich R, Rainen L (2013). Non-formalin fixative versus formalin-fixed tissue: a comparison of histology and RNA quality. Exp. Molecular Pathology 94(1): 188-94.

The method you use will entirely depend on what you want to use the samples for e.g. RNA analysis, electron microscopy etc.

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That's exactly why we use buffered formalin. Degrading formalin produces formic acid, which is the agent damaging DNA. Using buffered formalin neutralises the acid, and protects the DNA.

I use ethanol for freshwater benthics; but they are probably smaller organisms than you have?  We start out with 100% that gets diluted by the water in and on the organisms in the samples.  Once picked from the sediment we move to 70% ethanol with 1% glycerol.  We lose color (relative to formalin) but I have been at this for 2 decades and intend another 2 decades......I don't need the formalin exposure.  The glycerol gets sucked into the tissues if the ethanol dries out.

Buffered (hexamin) formalin fixation is good (but avoid breathing or getting on bare skin), and then - as suggested - change to ethanol. Or  - depending on samples - freezing is an option (but you need to be quick with the samples during sorting; specially good for algal samples including fauna). The best option is to try and sort live samples, but this is most often not doable.

Ethanol (70%) is best alternative to formalin.  Key for long-term storage is to have a good seal on containers and to check after a few months to determine if more ethanol needs to be added.  Problem I've found with freezing and thawing is it can create challenges in identification and there is the issue of having enough freezer space.